There are a few give-ins...simple social sign-up, tours, testimonials... but what have you found in your experience that really puts the icing on the cake when getting users to sign-up with the site. its a fitness site
37Signals did an amazing job with Highrise. They've been testing the signup page using different designs and sharing the results.
There are a number of ideas validated with a/b testing in these articles. One of them, a bit of a cliché:
Big photos of smiling customers work
Check the articles, and see if that's what you want. I am developing and testing a signup page right now. I think it worth doing a/b testing in the specific case of your page, I'm doing the same. When I have concrete results, I'll edit the answer to add them.
Personally whenever I see a site that tells me exactly what it does, looks elegant, and lists out exactly what it's capable of doing, and I'm interested in what it's telling me, I tend to register.
When people are informed without actually wanting to be informed, they're happy people, and are more likely to register to your site. I did a mini web-psychology experiment on something like this a few years ago. People are lazy, so your design has to help them!
A Login and Signup button should be displayed in an area that makes them very visible, sign up could start in the same window if you have the space. You should definitely inform the users what you're trying to do, what you DO do, and how you do it -- that definitely entices or un-entices users in to registering or going away. Then again, this should have some classic happy people, and other 'usual' marketing techniques to try and get them to be interested, it's no good losing customers!