Lending bitcoins seems pretty dumb to me. Whenever you give coins to someone else you allow them to fuck you over if (when?) the price changes in their favor.

If you still want to do it, make sure you are lending to someone who has a method to earn bitcoins (that isn't mining), regardless of the price. If they just need cash, they can screw you on a price change. If the price drops, they can pay you back faster, giving you less interest and also bitcoins that are worth less. If the price goes way up, they probably won't be able to pay you back and will just leave you with nothing.