To gain trust, endeavor to always be truthful, do not promise what you cannot deliver, and do what you say you will. This is not everything, but it sure will go a long way!
There's the old expression...Forgive and forget, well the forgetting part is not easy.. as far as trust goes, I think there will always be doubt once one has been cheated on... The question should...
Sometimes it just takes time. Sometimes they never gain trust back if you have given them a big reason or many reasons not to trust you. Also, there are some people out there who will never trust...
prove your trustworthy, or prove that you trust the other person. Tell them something you havent told anyone else (it has to be about yourself though other wise they'll think your a gossip). Be there...