I play mostly tournaments, so I'll defer to the ring game experts for their answers
My bankroll management strategy is to maintain at least 20 buyins for STT, and 50 for MTT. So, with $50, this is what I would do:
SNG: play anything up to $2 and don't move up until you have 20x the entry for the new level
MTT: play anything up to $1 and don't move up until you have 50x the entry for the new level
This is assuming that you are willing to drop back if you hit a losing streak. If your bankroll drops below $40, you should really move down to $1 SNG until you build it back up (to minimize the risk of going broke). If you're not able to do this, then you might want to play $1 until you have a little more of a cushion.
Tournament variance is rough sometimes (ok, *most* times) - don't fall into the trap of moving up too quickly after a big score - that win is going to be used to enter the next 50 (that you probably won't win).
I have a lot of respect for Chris, and for what he has accomplished, but I completely disagree with this last guideline. Essentially he's recommending that you sit down with no more than 5% of your bankroll and leave when you double it.
If I am in a game where I am making money because I am playing well and my opponents are weaker than I am, why would I leave? The combination of a profitable situation and winning table image is something to continue to exploit, in my opinion.
I think it makes more sense to set a stop-loss limit, if you are going to set limits at all (and I don't really like doing that either, as it generally results in scared play, which is -EV for me). Maybe once you double your buy-in, leaving when you lose more than 20% of your new stack is an compromise that is reasonable?
I think that it's important for any player - whether they play in ring, SNG or MTT - to be able to recognize when they no longer have an edge and to have the self-control to walk away from the game at that point. The size of the stack on the table is just one part of this, not the entire benchmark.
You should know which players you are winning against, and continuously gauge the situation based on this. For instance, if there are only 3 players at the table that you feel you can outplay and they all leave, it makes sense to consider moving - even before you lose your next pot.
Studying the table dynamics and deciding to leave when they change makes far more sense to me than quitting when you hit some arbitrary win number.
Obviously in SNG or MTT, I'm talking about understanding when you aren't on your "A game" and choosing not to start another one when you bust out - I'm not talking about leaving in the middle of one that is running