A really simple system, which can work for most but full time professional players, is the 100 BI rule. Meaning if you have 500$, you play 5$ tournaments or lower, if you have 1.500$, you play 15$ or lower and so on and so forth. With this system you can still play a 2$ tournament with an 800$ bankroll but not a 10$ tournament apart maybe from a very occational "shot". One of the advantages of this system is, it force you to move down during a losing streak, which tend to limit the losses.