If you're watering a friend's plants, or staying over and keeping an eye on their apartment while they're not there, we suggest you read XOJane's article, "How to house-sit like a non-asshole." Sure, some of the stuff is logical, but it also bears repeating. Some important takeaways:
1. Don't snoop. Period. This includes any drawers or closets. It's tempting, sure, but just don't. You never know what you'll find and it's a real invasion of someone's privacy.
2. Act at home, but not too at home. Using some coffee is fine. Drinking their unopened bottle of $100 scotch is not. Oh, and this should go without saying (please, please, please tell us it does), but the author advises that you don't use someone else's sex toys. Even if they're lying around. (Gross.)
3. Be cautious. Seriously. No candles, for instance. Just err on the side of caution. Extreme caution, as in apartment, meet kid gloves.