Watching and categorizing opponents will also help you in your play from the flop onwards. If for example you find yourself involved with an aggressive player who is going to be acting/betting after you then it may be preferable for you to check to him when you flop a good hand in the knowledge that he is almost guaranteed to bet or take a stab at the pot. In this way you get to win more chips off this kind of opponent as by betting you give him a chance to fold without giving you more of his chips.
Conversely, a meek or passive player in a pot with yourself may be prepared to fold a hand that may or may not be better than yours if you bet into him on the flop. The likelihood is that both of you missed the flop but by betting he may choose to fold and could even fold if he/she has hit the middle or bottom pair. Hence you can see that different types of opponents can be played differently from the flop onwards but to do this you must have paid attention previously so that you could categorize your opponent.
The play before the flop is much easier to conduct than at any other stage of a hand as you merely need to decide whether or not your hand is worth playing and for how much. Put simply - you can decide how much you are prepared to put in the pot and whether or not you are prepared to play for a raise or indeed for all your chips. However, once you reach the flop the fact that there are three exposed cards means that you now have to try and ascertain how good your hand is compared to any others who are also in the pot. This may sound straightforward and simple but in reality it is rather complex as there are almost an infinite number of combinations of cards that may now make your own hand seem quite weak.
So what are you supposed to do? Basically you have to try and determine what kinds of hands your opponents may be holding against you and to do this you need to try and utilize the information you have collected about your opponents along with recalling their betting patterns before the flop.
If you raised in early position and got two callers, then you have to ask yourself what kinds of hands your opponents would have called with. If this scenario is correct, it is most likely that they are holding a pair or two big cards (as these are the better type of starting hands) as they would be foolish to have called you with weak hole cards. Hence if the flop came up 224 off suit you would expect to have little to fear as these kinds of cards are extremely unlikely to be in your opponents hands. With a flop of TJQ with two hearts however, you may have to show some concern as you would have expected your opponents to have called with cards matching this type of flop.
Basically the way to proceed with a hand very much depends on the nature and texture of the flop. Because you will never have the opportunity of knowing for sure what an opponent is holding it is best to simply bet when you have some kind of hand that has a chance of winning and to check when you do not.
Essentially - when you think you are in front you should bet and if you think you are behind you should fold. You should also bet when you think the board has failed to assist your opponents and check or fold when you believe it is likely that it helped your opponents.
Perhaps what I am saying is that from the flop onwards you need to take care as a pre flop strong hand may now be rendered much weaker depending on what comes on the board. The more cards out on the board the more combinations of winning cards that exist and the stronger the possible range of hands that could be held against you arises.