Quote from Hydroblunt:
You don't put a ceiling on your potential, you simply WALK AWAY if you feel like you had enough. The risk/reward of the trading day changes quick after the first 1-2 hours, thats why there is a pattern of daytraders making great coin in morning and giving it back in the afternoon. Sometimes you know you had a good run and unless the day is special somehow and is very active, you're bound for a drawback. Just cause you can have a great run at the open, does not mean that you will continue on indefinitely to a gazillion dollars. On occasion you will, but most of the time it's just a good swing that is bound to retract. Don't be a pig.
Another technique is to allow a certain % drawback, like 15% before calling it a day. Unfortunately slippage often screws this up.