How much time are we talking about?
If 10 hours a year, just ask that he leaves it with full fuel and pay the extra to have him on the insurance. Good friends are more important than trying to recover $1000 or so.
That said, you don't want to set up a situation where it would damage your friendship. Keep this simple. Be clear up front - expected 10 hours, let you know if he/she's taking the plane, keep plane clean, leave with full fuel, etc. (Given he'll be leaving it with more fuel than when he picked it up, you should be covered for costs).
I like JScarry's take - if it's an acquaintance, $100 per tach time dry. Leave it with full fuel.
If it's 50 hours a year, then you need scheduling, agreement on who can take the plane when, etc.. I assume you bought the plane to be able to go fly whenever you wanted to, which will be compromised a bit with essentially a partner.