Seems like you've gotten a million different answers already so here's another for the pile:
What I understand is that keeping about 30% is the sweet spot for the best credit improvement, as others have said, in the end they want to milk you so it's good to show you're willing/able.
So when you go above 30%, you always pay back more than minimum. 1.5 or 2 times what minimum would be. Ideally not all to give them more time to milk you(cumulative interest).
When you are at 30% or lower, you pay the minimum and allow them to build the most interest and try to put some purchases on it to get it back above 30%.
If you do that, it'll show continuous use, a decent roi for them, and that you are able to pay consistently and above minimum so they don't have to worry about you.