for sng and cash games 5% of your br is the max to play with. for mtt only 3% of your br. It seems really tight but it has helped me
This man speaks truth!
Chris Ferguson also adds the suggestion of leaving any NL or PL game when the blinds reach you, if more than 10% of your bankroll is on the the table at any given time. I tend to follow that advice, because for me, when to stop has always been an issue. I like to be able to limit my losses.
http://www.thepokerbank.com/tools/
The above website has a nice calculator for telling you what level of NL cash game you should be playing with your BR. It also has a tool called the Fergulator that basically takes the above rules and spits out how much you can buy in safely for at your BR level and when to leave. That tool works for Cash/SnG/MTT calculations, and is really simple.
Anyway, there's a lot of smart people out there that can teach you about bankroll management. But just remember the golden rule: if you're playing a limit outside of your bankroll, you're really putting your entire roll at risk, due to variance. Be smart, and you'll go far no matter how large or small the roll you're working with.
And here's the original article by Chris Ferguson about his bankroll management rules:
http://www.chrisferguson.com/article-4
Hope you find this all useful. Good luck!