When to stop playing:

will sometimes make a decision to leave once I get to the next round of blinds if I have been doing well. It usually ends up being a good decision. Sure I might continue to win. But, then I could start losing as well. When you leave a table ahead of the game it is a nice ego boost that can take you through to the next session.

It seems like when you leave a loser the next session can be more of a struggle than if you leave a winner. Maybe it is just that way for a few of us. But, I know it is like that for me.

As far was when to leave a loser. Howard Lederer mentions that if you lose 30x the Big Bet you table image has eroded to the point where it is hard to come back. So, that it is best to exit at that point before you lose what little table image you have left. Maybe even sooner would be a good idea if you tend to start to tilt.

I have gone down as much as 50x the big bet in one session to come back to near even or just barely over.

But, that does take it's toll. Even though I might be able to put that session in the win column. I am usually so spent from that session that it takes a while to make it back to playing at a good level. Sometimes I even have to take a breathe for few days and refocus.
All of the above, and then when I get up, get out. Say I start with $20 on a .50/$1 Limit table. Then I have a good go of it and get up to $34. (not uncommen for me) I set a minimum that once I hit I am out. So at $34 I might say if I go below $30 I stop, period. The hard part is the discipline to follow your self implied limits. If you can do this consistently you will be better off in the long run. So much of poker is self discipline. Controlling your bank roll, playing smart, not going on tilt etc. I think that is one of the biggest things that seperate succesful players from the rest, self discipline.
Most of my playing experience is live play, but I learned a valuable lesson doing that. I would win early then fade as the day went on. What I learned to do was set a target, usually double my buy in. When I hit that, I would play as long as I was still winning, but I would leave after I lost the next pot I was involved in. Somedays I would keep winning and end up doubling again but even if I lost the next hand, I was still leaving the table with a profit. I was playing limit, so there was only so much I could lose in that first losing pot. If I were playing no limit, I would probably have left as soon as I hit my target and not ride out the rush. I certainly wouldn't put any more than half my profit at risk.