well... i guess there's not much to do, but the obvious:

1. make your site look clean and trustworthy (never ever ever play with pop-ups as this makes people think their credit card will end up with some russian underground webmaster scene)

2. make site look good and professional

3. add different sorts of "TRUSTWORTHY" banners on your Join/Buy pages

4. Put your contact info/customer email on the same page - that way it looks like if something goes wrong, there is actually a way to contact the owner (not everyone does it)

That's about all what I know...