Ok. I'm horrible with relationships so take all this with a grain of salt, but this is what I think I've learned about trust.
1. Trust is earned.
2. Trust is not binary. It doesn't need to be all or nothing.
3. Trust is topical. Its ok to trust someone in one area but tell them nothing of another area.
4. NEVER trust because you *wish* a person deserved your trust.
5. Remember that people are usually only as strong or as wise as they've needed to be to survive. Meaning that just because a person is nice doesn't mean they will be able to understand you, especially if they've never been through real trauma.
6. The flip side of that is: other survivors might be the best place to start -- it doesn't always have to even be surviving the same thing. Two hurting people don't equal one healthy person, but two veterans will understand war better than civilians.
7. A survivor friend and I have a term: "mutual blackmail." Of course we use it jokingly, but what it means is try to keep an equilibrium of the amount you share between yourself and your friends. Individual conversations will be different, but over the long haul it should not be just one person sharing and the other staying mum. Don't pressure them to share, but if they aren't sharing themselves don't over-share with them. They may be overwhelmed. (This doesn't apply to shrinks and other professionals, of course. Expecting the client to share the shrinks' problems would be inappropriate).
8. Not everyone I know agrees with the "mutual blackmail" concept, and feels rather that relationships that only flow one way are fine. I strongly disagree with this, but I wanted to say that in case you find it works for you. I don't see that as a friendship but rather a surrogate therapist, but with 7 billion people in the world every rule has an exception.
It'd be nice if I could round this out to a "TOP TEN" list, but I haven't learned 9 & 10 yet.