#1) The biggest and most common mistake that beginners make is that they play too many hands.  In a full ring game, with 9 or 10 players at the table, you should be calling or raising pre-flop an average of 18% of the time.  That means you fold at least 4 out of 5 hands, or play about 2 hands every time the Big Blind goes around full circle.  Playing inferior hands out of the hole, too many lower to mid suited connector hands, will get you broke in a hurry.  It is very important to realize the importance of correct pre-flop play.  Everything else in the game flows from the first hand decision that you make, which is the decision to call, fold, or raise pre-flop.  If you are consistently making the correct pre-flop decisions, then very likely that trend will continue in the post flop segments of the game, and that equals profits.

#2) The second biggest mistake novice players make is called chasing.  Chasing is calling bets on the flop, turn, or river, in the hopes of making a straight, flush, or hitting an over-card pair such as an Ace or a King, especially when there are no favorable pot odds to do so, which is most of the time.  Consistently chasing hands to the river will get you broke very fast.  You must overcome the inclination to chase in order to turn a profit, and the more quickly that you master that aspect of the game, the better.

#3) The third big mistake is that they don't bet aggressively enough when the time is right to do so.  If you have A,J and the flop comes A J 8, then you should be unafraid to bet the hand aggressively.  This is a big, powerful top two pair hand.  If you bet it, and get raised, you should re-raise.  If on the turn your opponent still wants to three raise you, then you should just call and slow down the betting, suspecting he has a set (two of a kind in his hand + the one on the board) of either Aces, Jacks, or most likely 8's.  At this point there is plenty of money in the middle already anyway.  But with a hand like that you are completely correct to try and get as much money into the pot as soon as possible.  You must learn to take advantage of the times in which you hit the flop solidly, and get as much money as you can from them.

#4) The fourth biggest mistake new players make, and a lot of more advanced players as well, is when they have a very good pre-flop hand, make a call or raise with it, and refuse to fold it after receiving an unfavorable flop.  An example of this would be: You are dealt a pair of Jacks in the hole, you appropriately raise with them from an early position, and you have one caller.  The flop comes A,K,4.  You check to the A,K, worried that the caller might have one of those cards.  Sure enough, he bets into you.  It sucks, but there is nothing that you can really do here but fold.  Most likely he has you beat.  If you call hoping to make a set on the turn or river, its a losing play.  Your odds of hitting it are too small.  The refusal to lay down good hands in these types of situations will put a serious hurting on your bankroll.  You can lose more money with your good starting hands than with your bad ones if you are unwilling to throw them away after an unfavorable flop.

I must also emphasize that when you are dealt a good pocket pair like 10's or Jacks, that you need to be careful if there is a lot of re-raising pre-flop from an otherwise non-overly aggressive player.  This indicates the likelihood that he has been dealt a higher pair, such as Kings or Aces.  It does and will happen, believe it.  So you must be willing to lay down your big pair if the pre-flop action indicates the presence of a better pair.  Again, good starting hands, especially big pairs, even though they are excellent, can wind up costing you more than lesser ones, simply because you are more excited and willing to put your money out there.

#5) New players call pre-flop much too often with one good card like an Ace or a King, and one bad card like a 6.  A,6 is a good hand if you are playing heads up, but a terrible hand if you are playing in a full ring or a six player ring(more on those later).  In a hand where you are dealt A,6, the six is called the "kicker".  A kicker is the lower card in an unpaired hand, or simply the other card that you have left in your hand if you make a pair on the flop, turn, or river.  Usually a kicker is thought of as the lower card in your hole card hand, but if you have A,6 and a six hits the flop but not an Ace, then the Ace would be your kicker.  So the problem with playing Aces, Kings, and Queens, with poor kickers is that they usually get beat be Aces, Kings, and Queens with better kickers. 

In Texas Hold Em the winning hand is the hand with the 5 best cards out of the seven you can use by the time the river is dealt.  So if the flop comes: A, 10, 4 and you call a pre-flop raise with A,6 and your opponent has A,J, he is way ahead of you.  Now, if you call to the river you are in it for 5 bets. Say the turn and river come 8-9:
                                            Your hand is now AA, 10, 9, 8.
                                            Your opponents hand is AA, J, 10, 9.

He has you beat and it cost you a good deal.  This is also called being "out-kicked".  A,9 is the first decent Ace hand, but I would still fold it pre-flop to a raise from a normal player, and throw it away from front all of the time, from the middle sometimes, and try to raise from the back with it, if the action came to me unraised.  Occasionally I might also raise from the middle with it.  It is one of those starting hands whose value depends on the circumstances very greatly.

Pre-flop, if your position is decent, you should start thinking about calling a single raise with A,J, and fold to a raise up to A,10.

#6) The sixth biggest mistake novice players make is falling love with suited connectors and looking at them as if they have more value than they do. 

Lower ranking suited connector hands: 2,3  3,4  4,5 and 5,6 are practically useless and should be consistently thrown away pre-flop from any position. 

Mid level suited connectors: 7,8  8,9  and  9,10 have some value in different situations, but not as cards that you can regularly call with.  You should throw them away more than you play them.  In cash games, their main value lies the fact that they are good hands to call or raise with when your table image is that other players see you as being too tight a player.  If they see you that way, they will fold to your bets and raises when you have good hands like A,K, and you therefore can't make a decent profit off your good hands.

If that's the way it seems to be going-then raising with 7,8 suited is a good way to make them have less respect for you.  Even if your raise is called by A,Q, if the flop comes in your favor, 6-7-8 for example, you can likely make them pay all the way to the river.  If the flop comes high, A-10-8, and they call your first bet, its pretty obvious your beat and you can check and fold on the turn.  As you fold make sure that you click the "show hand" button so they know that you raised from middle position with 7,8 suited.  Either way, its going to pay off eventually, if you play those types of hands for that reason.  If you play them regularly as you would play A,K, then they will you lose you money, and that's a fact.

#7) New players don't throw away small pocket pairs enough.  You can play small pocket pairs occasionally, but remember that your chances of hitting a set are only 1 in 7.  Most of the time you will miss your set.  So a pair of 2's through 8's, without hitting that set, usually are going to get beat.  9's are the first really good pocket pair hand, which you should raise with pre-flop if there hasn't been a raise already. 

The worst time to play a small pocket pair hand is to a pre-flop raise.  A pre-flop raise indicates the likelihood of a big Ace hand, such as A,J.  To call a pre-flop raise which has already been called or re-raised, with a small pocket pair, is even worse.  Now you are looking at the likelihood of a large part of the top end of the board, Ace through 10, being covered.  Your 4's are just not going to hold up.  You might call with 9,9, or 10,10 -it depends on who is doing the raising and what your perception of their play is.  But if the flop comes high and they come out betting, you should fold.  If you have pocket 9's or 10's and the flop comes out:

                                        6, 7, J    then you should be much slower to fold to a single bet.  A lot of good players would raise in this situation.  You don't want to be a wimp, and there is a good chance that your oppenent's A,K, A,Q or A,10 has missed the flop.  However if there is more than one other player in the hand, and they are raising and re-raising each other, you should fold.

#8) New players don't pay enough attention to the post-flop betting action.  Even if you are making all of the correct pre-flop decisions, if you are not allowing the post-flop betting action ("action" is just a word for what other players are doing-calling, raising, or re-raising) then you can not make correct post-flop decisions and you will lose money.  The post-flop betting action is your #1 source of information for how you stand in relation to other players in terms of your respective hand strengths.  You should be watching it and taking your cues from it.  Say you are in a hand with J,10 off-suit.  The flop comes 9-9-J.  You bet it and are raised.  You know that it is likely that the raiser has a 9, assuming he's a normal, non-overly aggressive player.  But say you bet it, and are just called.  In this case the caller may have K,Q hoping to make a straight, or they may have K,J in which case they have you beat but are afraid that you are betting with the 9.  They also might have a 9 in their hand, correctly thinking that they have you beat, and are "slowplaying" you.  Slowplaying occurs when a poker player knows that they are in a superior position, but only check or call in order to extract more money from the hand. 
 
So you have two different sets of possible scenarios just by the difference of a call or a raise.  If you are not paying attention to these things, and deducing what scenarios exist based on the play of the other players in the hand, and play only on the strength of your own hand, then you can't make enough of the correct post-flop decisions in order to turn a profit.  So watch the action and become practiced at clearly thinking through the different scenarios that it indicates.

#9) New players tend to give too much weight to one particular element which informs decision making, rather than balancing all the elements together.  This is actually an advanced beginner mistake to make, where you've come to the point that you've played enough to perceive and bring a lot of different elements together, but don't have enough experience to consistently balance them in the right way.  For example, you might pick up on the fact that a certain player likes to raise from late position with mid-level suited connectors.  You're in middle position with Q,10 suited and he raises your call.  You think you've got him beat so you re-raise.  He maxes it out and you call.  The flop comes:

                                          Q-8-4   You've hit top pair with a moderate kicker-You bet and he raises, you re-raise, and he maxes it out again.  As your calling this last bet it begins to dawn on you, after being into the hand for 8 bets, that this guy very likely has the goods, either A,Q,  KK, AA, or possibly even QQ.  Your single observation that he likes to raise pre-flop from a late position with mid-level suited connectors has blinded you, and lead you to act in a way in which you otherwise would not have acted.  So you have got to be careful with the inferences that you make, and how much weight that you assign to them in relation to the general things that you know about the game, and the specific things that you know about the particular hand in question.

#10) New players are too predictable in their play.  In the very beginning of your playing career, when you are trying to build good pre-flop and post-flop habits, its OK to be predictable.  Its a much better mistake to make than the usual ones such as playing too many hands or chasing hands.  But before too long you are going to realize that if you never change your style, your opponents are going to be able to read you too easily, and fold to your bets when you have good hands.  If you can't cash in on your good hands, then that becomes a real problem.  So eventually you are going to have to throw in a change up, like raising from P5 with mid-level suited connectors, or something of the like.  There is a fine line between making this kind of play too often, and making it just enough so that your opponents can't easily read you.  But in order to maximize your profits on good hands, you have to find that correct balance.