While Stephen's method would work you have to be careful with it as some search engines would consider it as Spamming (in the search engine sense). While I'm sure you aren't planning on using this in a deceitful way, the Search Engines have no way of being able to tell your intentions. If they do deem it as Spam they might de-list your site from their index, which if you aren't in there anyways might not be that big of a threat.
My suggestion (not knowing the type of site you have) would be to select a few pieces of content that are optimized for your most important key words and make them available to everyone including the Search Engines. This could help get new users to register as they would be able to sample a small sub-section of your content before they have to register.
Hope that helps,
--James
