The most important is to set up some rules for yourself and then follow them. Especially rules for when you need to move down. A daily stop loss is also a good idea and more important than in live poker, because you can play such a large volume online. So if you run bad and start to tilt, it can get really expensive in just a few hours. For me some reasonable miminum numbers are:
* Full ring cash games 20 x a full 100bb buyin
* 6-max cash games 30 x a full 100bb buyin
* Single table SnGs 50 x the buyin
* 45 man SnGs 75 x the buyin
* 200 player MTTs 100 x the buyin
* 500 player MTTs 150 x the buyin
* 1.000 player MTTs 200 x the buyin
The exact numbers can always be discussed, and if you want to be more conservative, that is fine. But if you stick to the above, the worst, that will ever happen to a winning player, is, that you might need to move down.