Have a look at StackOverflow or pretty much any "social" site, like Facebook or LinkedIn. The registration process is as short as possible. Then, after they are registered and signed in, prompt them to flesh out their profile with more information. Remember that you need to entice them by doing so. SO gives out badges to users who filled out all fields. LI keeps a percentage score of "profile completed". Those things nudge users to want to fill out their profile.