Try to keep it above 75% - it takes a while to recover. You can only upvote 8-12 times a day and keep your power. Only time allows your power to go back up. The lower you go - the longer it takes.

I got down to 40 and then only upvoted a few times each day for about 2 weeks to get back up above 80%. So you have to be careful not to vote to much and make your votes count.