Leave No Trace
If there is one thing to take away from these tips, it should be this: leave the place as clean as possible when you go. The last thing you want is for your host to have to hire a cleaner after your stay.
On the last day, triple-check the bedroom and bathroom for your belongings, and strip the sheets and put them in a pillowcase at the foot of the bed. Do a quick wipe-down of the counters in the bathroom for toothpaste or hair. Make sure all your remaining glasses in the sink are washed. And, of course, return any items to their place, like a book you took off the bookshelf.