1. You're underestimating the power of short-term variance. Someone who's thoroughly beating the game can be a loser over 3000 hands.
2. It's all one big session.
3. Quit when you're feeling you're not playing well, or you want to quit, or the game starts to suck (an influx of TAGs often kills a game). If the TAGs arrive and you don't want to quit, find a better table.
4. Think about your opponent's ranges.
5. Think about your equity.
6. Don't think in boxes (Top pair isn't worth more than $20, for example). Think about ranges and equity.
7. If you're following correct bankroll management guidelines, you shouldn't be scared to put money in the pot if you think your equity is good.