I like to use the 30/25 rule. I move up when I have 30 buy-ins (and that I feel ready) and drop back down a level if i only have 25 buy-ins for the higher level. I just drop back down whether it is because of bad luck or poor play.

This way if you do drop 5 buy-ins moving up, you can figure out whether you were outplayed or it was negative variance. It also forces you to make 10 buy-ins at the lower level again and you'll have more confidence on your next try.

Obviously you don't have to use the 30/25 rule, you can do 25/20, or 35/30 depending on your risk tolerance, the level you play at, and how bad streaks affect your play. It also depends on how many tables you play, if you're 4+-tabler I think you should have more buy-ins for a buffer, but you don't need as many if you play only 2 or 3 tables at once.