Okay, being in this situation myself I'll give my two cents...
Like any relationship of any kind, trust and communication are key. How my bf and I make it work is we keep in constant contact. If he has to go somewhere, he texts me where he's going and when he will most likely be back. Something happens on my end I tell him as well. We even share our dreams we have, and though I've had some dreams involving other people, not once was he ever mad for it, saying "it was just a dream, but in the here and now it's us." I think things like that build trust.
I admit in the beginning I was real skepticle and insecure, sometimes I can be still, depending. But he has yet to do anything to break my trust and I told him, he ever wanted to stray, just have the decency to tell me first. He doesn't want to stray though, and despite the many home issues I have that seem to keep us apart, he still wants to be with me.
Maybe I'm just lucky, I don't know.
But if someone breaks your trust, why keep giving them chances? A guy I tried to see before my boyfriend of now did just that. Lies out the wazoo, then I found profiles of him online talking with various girls I didn't know, some saying they loved him and other things in a gf/bf nature. I dumped him and not once did he contact me after I stopped contact altogether, and he easily could have. So well all know what he was about. *cough*dog*cough*
And Miss Jane: I tend to have this saying:
"Fool me once, shame on you. Fool me twice, shame on me. Fool me a third time, I'm kicking your ass."