Ive been having a similar experience since moving up to 25NL. I managed to break even until yesterday when I lost a few buyins and now im down to $440. I figure I need $200 to play safe comfortable 10NL so ill probably stick at 25NL until I hit $350. Hopefully it wont go that far and ill start winning again first .

The staggering of the buyins means I have quite a comfortable window to be able to lose in before I move down, and still be slightly overbankrolled for 10NL if I need to move down.

My suggestion at small stakes would be to move up when you have 20 buyins for the next level, and move down when you are down to 30 or so buyins for the previous level. As you get higher in stakes you will probably find you need a relativly bigger bankroll as your edge will get smaller. You might want to get to 30 or so buyins before moving up at high stakes but i havnt actually been there so thats just a guess.