Send a thank you note
In this age of cellular phones and email, a hand-written note--even a brief one--carries special meaning. So when you've returned home, buy a card and a stamp and express your gratitude in writing. Your card may just end up on your host's refrigerator, serving as a reminder of your stay--and your good manners. If you can't get your act together enough to send a card, an email or telephone call are better than nothing, and usually sufficient for close friends and family.
If your stay lasts more than a few days, youmight consider sending a gift as well as a card. If you do plan to send a present, try to figure out the perfect gift during your stay. What is missing from the host's home? What would match their tastes(or at least their color scheme)? A gift isn't a requirement, especially if you brought a housewarming present, but it's a nice touch, especially if your host went up and beyond the call of duty.
Now your duties as a guest are complete, and it's time to consider when you can return the favor by playing host yourself!