Good question. Probably no easy answer.
I'd suggest:
* being very comfortably bankrolled. There are more than a few good players who advocate 100 buy ins. Way more than the 20 buy ins you'll see mentioned.
* asking yourself where your edge comes from, which will give you confidence in your game. Put it this way, if you can't
say where your edge comes from then you have to seriously question why you're playing in a particular game
* related to the above, spending some of your poker time on studying the game, as opposed to simply playing a ton of hands so that when the time comes to ship the chips into the middle, you're doing so with confidence, rather than guessing 'n hoping.
* playing the game that's best suited to your personality, which is something that perhaps doesn't get talked about as much as it should. If you're the type of guy who likes to get in there and mix it up with the bluffers and out-psych guys, then No Limit is the way to go. If you're more a maths/technical kind of guy then maybe one of the Limit games is more your thing. (In before 'limit is dead'
)
* remember, you're not primarily trying to win money. You're trying to make good decisions. Do that often enough and $$$ will follow.
Good Luck.