I just learned this one last week:

Have a friend go to your site, navigate through your preview galleries, and try and buy a membership. Make sure it's someone who isn't all that familiar with porn sites, the Internet, or e-commerce sites.

Encourage them to ask their questions outloud, but don't guide them in any way. By asking questions, it'll become more clear on how they might be confused by your site or method of navigation.

My friend pointed out that I was missing a "join" button in some pretty key areas. Since I added it, sales have jumped.

Dzinerbear