This question comes up time and again.

How do I move a website to a new URL and not lose search rankings?

Aaron has the answer, and some great tips for moving a big site.

“The best way to permanently move a site is to 301 redirect it. If you have a small site you will likely see few small changes with your rankings. The bigger your site is the longer it takes to move and the more drastically the shakeup will be.”

Everything else you need to know about 301 redirects:

•  URL Redirection Explained – Wikipedia
• 301 Vs 302 – Matt Cutts
• 301 with mod_rewrite – V7N
• 301 Permanent Redirect, Not Meta Refresh – GNC
• How To 301 Direct On PHP, Coldfusion, ASP.Net,  And ASP.