Google webmaster tool blog introduces how to change the domain name of the website, how to keep the original PR:

Before you change the domain name, try a directory or sub domain to see if these pages are included in Google.

Instead of turning the page on the original domain name to the new domain name, it turns to one to one.

Do not change domain name and revision at the same time.

Using Google webmaster tools, check all internal and external links, trouble your friends to modify the link to your new domain name.

Six months later, you will discard your original domain name.

Create website maps, add new domain names to webmaster tools, and continue tracking old domain names to see what's wrong.

Meanwhile, please refer to:

Google printing and changing domain name

Tell Google your domain name is changed.

Google strategy for blog moving