http://uxdesign.smashingmagazine.com/2008/07/04/web-form-design-patterns-sign-up-forms/
The article discusses a survey that was conducted to observe trends in the UX for signing up for services provided by a website. Much of the covered topics involve what makes the process of registering with a site more welcoming, less distracting, and less strenuous on eye movement to navigate the page.
A lot of changes over time have been focused on simplicity and placement rather than trying to draw too much attention. The links to the form are usually in common places like the top-right corner or the form itself will be displayed in part on the homepage. There is also a lot less required information or even optional information to help streamline the process of signing up and allow the user to add more to their accounts later if they want to.