There are three decisions you will typically have to be concerned with preflop. The first is how much should you raise when you are opening the action. For the most part, this will depend on what your goal with raising is. Are you looking to build a pot or do you want your opponents to fold? If you're a loose aggressive player, you probably want to see a lot of flops and induce a lot of action and thus a smaller opening raise such as two to three times the big blind is better.
If you're a tight aggressive player, you probably want your raises to generate respect and build larger pots with your premium hands if people do choose to play with you so three to four times the big blind is a good starting point. But bear in mind the players in front of you and how big a raise they are willing to call with marginal hands. As always in poker, be observant.
Punishing limpers
The second decision is how much to raise when there have been players that have limped into the pot. Often, players have a hard time folding once they have put chips into the pot so you need to be aware of this when making a raise in these instances. Usually players are limping in with hands that they want to see cheap flops with. How much you raise should be a matter of whether you want them to call or not. If you have A-K, for example, you probably want to raise enough to take it down right there. However, if you have pocket Aces or Kings, you probably don't mind a call from a hand like 8-7 suited or pocket Fours and should make a smaller raise.
The last type of bet to be concerned with is how much to raise when you are already facing a raise. The problem most people make here is not raising enough or raising too much. Not raising enough invites action you don't necessarily want. You don't want to give a player with A-T 3-to-1 on their money to call you when you have pocket Kings. Raising too much can make the pot too large and will make your post-flop decisions extremely difficult.
One last consideration you should account for is how much do you need to raise in order to accomplish your objectives? If raising to just three times the big blind will get everyone to fold, then there is no reason to raise to four or five times the big blind. This is why late in tournaments when the blinds are so high relative to players stack sizes that you will see a 2BB to 2.5BB raise working effectively.
Betting the flop
There will be varying opinions on how to bet on the flop, but I believe the best approach is a consistent one. What I mean by that is to make the same type of bet every time you are leading the action. The amount doesn't matter as much as the fact that you are consistent, although it is probably better to use a smaller amount of half to two-thirds of the size of the pot than to bet the size of the pot.
A smaller bet of half to two-thirds of the pot will allow you to fold those hands when you get reraised but is also a large enough bet that you will get respect. It's also a small enough bet that it's an action generator, meaning that people will continually test you by calling you down or raising you in improper situations (when you have the best of it).
But this does not mean you should always bet this amount. Certain situations will warrant making a different bet amount. For example, say you see a flop with an opponent and a half pot bet would be 40% of your opponent's stack. You know that you will not fold if he moves all-in over the top of a half pot bet, so rather than leaving yourself with a decision you move all-in and put the pressure on him. In that type of situation, it's perfectly acceptable to bet something other than your standard bet.
How much to reraise
When it comes to reraising a player, how much you bet will depend primarily on how deep your stack is. If you have plenty of chips behind you, then you should be making a raise that will best accomplish your objectives. If you want a call, you want to raise the maximum possible for that particular opponent that you believe they will call. If you don't want a call, you want to raise the minimum possible that you feel they will fold to.
I still think a standard amount works best in these instances because you are then able to bluff raise and value raise without giving your opponent any information. When you start varying your raise amounts you always have the potential for giving your opponents information about your hand. What amount you raise is up to you, but I typically raise two times their bet amount. So if they bet 1,000 into a 2,000 pot, I will raise it 2,000 to make it 3,000 to go.
Bet-sizing on the turn and river
The turn and river are a bit different than the flop. On the flop you are trying to achieve a particular goal without giving away any information about your hand. Once you have hit the turn though you have a lot more information and now the goal is to win the pot. And if you have the best hand you want to make it the largest sized pot possible. The answer as to how much to bet in these situations is 'it depends'.
What it depends on is your opponent and the likely hand they are holding. This is where careful observation of how they play and what their tendencies are will come into play. Are they the type of player to only call with a draw on the flop? Are they the type of player that will call with any piece of the flop, whether it be bottom pair or top pair, weak kicker? Use the information available to you to help you determine how much to bet. If you have a very strong hand that is virtually unbeatable, you want to make a bet that will get called or will induce a raise. If you have absolutely nothing but think you can get your opponent to fold, you need to make the bet that you think will get him to fold.
Protecting your hand
If you have a strong hand but one that is vulnerable, you want to make a bet that will be large enough to get value for your strong hand but at the same time will make it an error for your opponent to call. For example, let's say you have A-K on a Kc-Jc-6s-2h board. Your opponent just flat called you on the flop and you feel he has either a straight draw or a flush draw or both. Your hand is probably the best but it is vulnerable. You don't want him to fold but at the same time you don't want to make it easy for him to call. A bet of approximately 75% of the pot in this instance would probably accomplish both of your objectives.
The bottom line when it comes to bet sizing is that there is no fixed amount you should bet. You bet the amount needed to get the job done. Being consistent with your bet amounts will help and I recommend doing so preflop and on the flop, but in the end it's all about getting your opponent's to make mistakes. And you can do get people to make mistakes with any size of bet as long as you are paying attention to how they play.