How do you gain someone's trust?
Slowly. It takes time. Don't do anything questionable. :o
I think this will help http://www.ehow.com/how_4723805_gain-someones-trust.html#page=4
Prove to them that you can deserve it.
Be trustworthy. If you say you are gonna do something, do it! If you say you won't, then you better not.