The first thing is to understand that trust is not given......it is earned. No one can say "You have to trust me, or we have to trust each other" the reality is that trust can not simply be given that easily. Trust takes time, and is something both of you have to work on.
The fact is you both have to give and take here, of course trust isn't something tangible and the fact is you never know if someone is breaking the trust, until the damage is done.
The first step is to set down some ground rules.
1) Never go out to spots where betrayal is more common.
2) Be open and honest with each other, honesty is the best way to build trust. If something happens, be honest with your partner.....this way you both know if something happens instead of hiding it.
3) Don't keep secrets, don't encourage secrets. Open the phone bills together, keep open access to checking accounts. Manage the money together and if there is money missing, talk about it and supply proof of what was spent. Set up your histories on computers so each can view the others actions without fear of being sneaky or hiding it.
4) Talk about your feelings.......if something bothers one of you be honest. Even if it may hurt the other persons feelings a little bit. Being open and honest about everything (And I mean everything) then hiding and lying will find less of a hold on your relationship.
And the top thing you can do is set up marriage or relationship counseling. A therapist can help you both deal with the problems that originally caused a problem and how to deal with these issues in the future.