Good credentials (i.e. someone I trust trusts them) initially, then good experience with them. Trust deepens with time, but it always is interesting to me how one crack shatters it.
I am always giving people the benefit of doubt and trying to see things from their perspective, which has lead me wrong many times. I don't seem to protect my "outer" world.
But that's not been true with my heart which I seem to guard - the only person I truly trusted with my innermost feelings was my husband. And he was the only person I knew who celebrated me for who I am.