There are many factors that affect your decisions at the poker table. Obviously, the cards you're dealt often dictate whether you'll even consider getting involved in a hand, but they're just one part of the equation.
Once you're playing, things like your physical state can also affect your play. If you're hungry, tired or even a little drunk, you're not likely to play your best, and your decisions may not be as smart. Emotion is a factor too. When you're winning, you often feel like you can make any hand you need to win a pot. When you're losing, however, a continued string of beats can seem unbearable. This can lead to tilt and keep you from playing at the top of your game.
Using this kind of information against your opponents is one of the keys to becoming a winning player. If you know they're a little tired or a little tilted, you can determine if they're likely to call a well-timed bet or bluff.
For example, let's say you river the nuts and want to induce your opponent to put more money into the pot. What do you do? The answer often depends on your opponent, and what size bet you think you can get them to call.
One factor to consider is how well your opponent is doing in the game. Are they winning or losing? Let's say you know a player sat down with $500 and they now have $710. This player is much more likely to call a bet of $140 to $170 on the river than a bet of $220 or $250, because the additional money pushes them through what I like to call their "pain barrier".
For your opponent, calling a $170 bet means they'll still be up for the session, even if they lose the pot. Calling $220 or more means they'll be down. For many players, the psychological difference between these two scenarios is huge, even if they don't realize it.
There are other factors that can help you manipulate your opponent's pain barrier at the table. For example, a player who is sitting with case money (i.e., they don't have any more money in their pocket or stored in the cashier) is much more likely to be pushed off a hand by a big bet if they're holding any kind of marginal hand. The pain barrier becomes even more effective if you know your opponent is about to quit the game. They might have had a phone call from a spouse or be going to dinner soon; then the $250 bet in the situation above works even more often, as most players don't want to finish their session on a losing note.
Obviously, using the pain barrier won't work every time but, if used properly, it's a great weapon to have in your arsenal.
[From Full Tilt Poker]