Keep it personal
Contacting someone saying simply "Hello. Can I stay at your house for a week?" is unlikely to elicit a positive response. Introduce yourself and your plans. Where possible make the person feel you've chosen them for a reason.
Always reply
If you request to meet someone and they send a personal response to say they won't be able to make it, return the courtesy with a reply rather than just moving straight on to the next person.
Communicate
Keep to your plans. Don't leave your host waiting for you. Don't pull out at the last minute.
Give a little
If you're staying at someone's house, bring a gift (maybe something typical from your own country). If they're showing you around town, buy lunch or drinks if you can, and always pay your way. Many guests offer to cook their hosts dinner; cleaning up after yourself should go without saying.
Be courteous
If staying at someone's home, do not use it as a base to party with other people. Fit in with host's schedule. Don't sleep in for hours. Don't overstay your welcome.
Socialise
Make sure you spend time getting to know your host. If you're just after free accommodation or a tour guide, you've got the wrong idea.
Consider a skill swap
One way to give something back is through a skill exchange. Offering a dance lesson or DIY expertise can enhance the travel-networking experience and increase the chances of being hosted.
Keep in touch
Lots of hosts don't travel themselves, but open their homes in order to make friends around the world. Don't disappear off into the sunset when you leave. Drop an email every now and again. If they've got on a travel-networking site, it's highly likely they're on Facebook, MySpace or the like.