Here's a must-read article -- BestBuy saw a $300 million increase in annual sales via their online store by not requiring users to register at all to buy products (this means they would have to enter their address, etc. every time they bought something). If that's not enough,
Later, we did an analysis of the retailer's database, only to discover 45% of all customers had multiple registrations in the system, some as many as 10. We also analyzed how many people requested passwords, to find out it reached about 160,000 per day. 75% of these people never tried to complete the purchase once requested.
This speaks volumes against requiring user registration at all. It makes sense for something like Amazon, where customers frequently re-purchase things, but not so much for... well, most other online stores.
If you must have registration, consider the way Orbitz does this. You enter your info, and can optionally enter a password, which will let you log back in and view flkight updates, etc.
You can still collect a lot of user information by correlating names/addresses, o you're not losing much business intelligence by not having users register. If you really want users to register, incentivize them by saying the next purchase (of over $X) is 10% off if you register, or something like that.