DebianHelp has made an excellent article about how to move from Sid/Unstable to Sarge/Stable.
As they mention:
This article will outline some of the details of that move and how easy it was for those that have never gone through such a move. If you're an experienced GNU/Linux user, you're invited to share your own thoughts, clarifications and tips on managing a desktop machine.
And if you're a new GNU/Linux user — or worse, a Windows user — read on to see how computing can be made (relatively) simple.
Read the full walkthrough here.