Learning to recognize when I'm going on tilt has been one of the most important processes in my becoming a better poker player. I found that underplays threw me off my game far more than anything else, so I've worked hard to take my chances when I get them, and quickly move past it when I don't. It wasn't until I lost about $10 (mind you, my bankroll was $20) after getting nothing out of a straight flush that I realized how much such things affected me, and took steps to address it.

Obviously there's a huge difference between $10 and a few hundred thousand dollars, but the same core concepts apply. It's difficult for people who haven't played a decent bit of poker (even at very low stakes) to understand how psychologically draining it can be, and how much the player's frame of mind affects the game.