You can start by implementing either Analytics or Target on your web page. I've removed the link because it's not relevant, since you have no products implemented pertinent to this forum.
If you're getting a 60% drop off between registration and purchasing, you may want to consider revamping the UI and the process it takes to register. Displaying one registration process to half the visitors to your site while displaying a different process to the other half will help you determine the best and most efficient methods to obtain conversions.
If you're genuinely interested in creating an optimal customer experience on your site, I would highly recommend looking at the book Refining Design for Business by Michael Krypel. It outlines the exact issues you're facing with solutions and actual examples large organizations have employed to increase purchases and revenue.