Honesty and consistency are a good place to start.
Edit: The external packaging is not particularly important to me. If someone is rude, blunt, and says terrible things, but after observing I can get an idea that their motivations are good, I can trust them. It might take me longer to recover if the person has all the social graces and seems really kindhearted, but then unexpectedly stabs me in the back. For me consistency has to do with the presentation of the person being authentic. There is also always the possibility for an individual to find patterns in a variety of people that either support or undermine trust. I have found that image conscious people tend to be the most critical and undermining of me, so when I get a whiff of that, I tend to back away a bit.