[foal]

I don't know that it's that much of a problem to quit when you're ahead. If you're scared of ruining your winning session then you might start playing sub-optimally. Obviously the best way to go is to play without thinking about whether you're "up" or "down". I've found that when I set goals for myself like "I'm going to see what winrate I can achieve over 10k hands" I start playing poker for poker and not for money. Playing sessions become less significant, because they're just a small part of that larger session.
Poker is a sick and brutal game if you care about your results in each and every playing session.