I'm a fan of sitting down, dividing free and paid <i>users</i> in your market, and then mapping the features to the offering such that folks will shake out where you expect them to.<p>I don't explicitly say "Free for parents, $30 for teachers / Fortune 500 companies / parties" anywhere on my site, but that is essentially what the 15-card free trial cap does.<p>Your free users are not an undifferentiated mass with a static probability of converting. They've got different needs. Figure out whose needs you can satisfy while charging money for. Do it. Then, for folks you can't conveniently monetize, figure out how you can derive business value from them anyway.<p>(Stupidly simple: ask them to link to you in your signup mail. This won't take you five seconds to implement, but it works, and you can use all the SEO help you can get to attract more paying users.)