You've heard it probably a thousand times: poker is a game that is all about making good decisions. You've likely hammered that thought into your head, or perhaps written it on a sticky note and posted it on your computer monitor, and if you have given it a shot, then you have learned something important -- it can be really hard to do.
One of the hallmarks of the successful poker player is not that he or she makes good decisions from time to time; it is that he or she makes good decisions all of the time. This is, of course, a slight exaggeration -- everyone makes mistakes now and then. However, you cannot use this as an excuse for bad decision-making. If you are making mistakes and losing at poker, it better be less then 10% of the time if you wish to be profitable. The pros and other really good poker players can afford to be wrong every once in a while, because they are right so often.
This is why discipline and patience is such an important key in Texas Hold'em and other forms of poker. You may start off in a tournament making the right decisions and then allow something to throw you off your rhythm. It could be practically anything: you may have gone on tilt after getting sucked out of the game, you could be paying too much attention to other things rather than the game at hand, you may just be getting bored from folding junk -- whatever it is, you need to recognize that you have stopped making good decisions as soon as possible.
Once you have stopped consistently making good decisions, you need to do something to start making them again. Tighten up, sit out a hand or to, whatever it takes; just stop taking action until you can be reasonably sure you are back in your game. Consistency is key.