Ask HN: Gaining users trust via face to face marketing?

Ok so here we go... my startup adsreloaded.com has had interest from several developers but it's been surprisingly hard to gain users. I literally pay people to download iPhone apps, but marketing is still a big beast. (Yes I realize there is the argument of people who can afford iPhone's don't need a few extra bucks... point taken, but I think there is a huge audience still especially the younger age group)

Here are a few ideas I have had that I think could work and was wondering if people have had success doing it:

1) Rent out a mall kiosk (Expensive) 2) Rent out booths at flea markets (generally not my target audience) 3) Try and find tech expos and grab a booth there (Are there any resources for tech expos in the bay area?)

Building trust seems to be a big issue for users... I could even pay people right there in cash if I was physically at a booth.

What kinds of direct marketing have you found to be the most effective for your startup?