From Social Patterns
The easiest way to talk like a person (in the user interface copy of your site or application) is to adopt a conversational tone.
People reading impersonal text on a screen will remain disengaged.
Use this pattern when writing copy for a social site, including instructions, errors, and other messages coming from the system itself and addressed at the reader, visitor, or member.
Resist the urge to write like a grad student or a bureaucrat. Ask yourself if that’s really how you talk. Read any copy out loud and strike out anything that feels awkward to say. Try speaking the text out loud to another person to see how it feels in your mouth, hear how it sounds aloud, and observe how the other person responds.
Despite what your English teacher may have told you, it’s OK to use contractions, to split infinitives, and even to start sentences with conjunctions. Just make sure it feels natural.
Be careful to avoid obscure slang unless you have already established confidence in the minds of your users to the extent that they are willing to rely on context and gist to follow your meaning.
Don’t mistake being cute for being real.
A conversational tone provides an opportunity for your site’s visitor to respond as if really being spoken to by another person. This receptive state of mind permits the reader to enter into a dialogue with the site and reinforces the feeling that the site is made by people and not machines.
Flickr's terms of service (see above) epitomize this plainspoken approach, particularly the "Don't be that guy" comment.