I've read a couple places (can't remember where) that a good rule of thumb is to take out half of your winnings whenever you double your br.
Agreed, i do 20-30% though. While i dont really need the money for anything important i find that to actually see that playing poker is giving me something back then i encourage myself to continously strive to get better and improve the level i play at.
As for stepping up levels i use 20 buy ins. Then when i feel ive ben playing well or had a ncie run of cards i try just one table at that improved level. I play my normal game and see how i play. If its succesful i start playing it a touch more often. As i see good results i start making that then level i play at or if i have trouble i move back down and reexamine all aspects of my game (even watching a table at the higher level helps )