Lawrence Boxer talked about cashing out when you are up money and 'locking in the profit'. I understand the thought process but you really aren't locking anything in. If you are ever going to play again, then you will be playing with that money again. A session is a pretty artificial grouping of single events. If a daily profit is your goal, then go ahead and lock in those profits. If there is some magic formula to knowing that you have hit the high point of the day, week, month, etc I'd like to know it. The hand you play after being up $500 has the same expected value as the hand you play after being down $500. The only time you should 'take the money and run' is if you are playing a negative game. Then each hand you play has the same negative EV.
Negative runs and positive runs are part of the game. Unfortunately, you don't know when they will happen or how long they will last. I firmly believe that the hand I'm playing doesn't know what the last hand(s) were and there is no relationship between the previous hand(s) and the current hand.