Migration from one server to another - best practices?

[Dennis Carr]

I'm about to do a migration from one server to another - old server runs 
Debian Lenny, new one runs Squeeze, both with respective current versions 
of postfix.

Long and short is that I'm basically preparing to migrate everything, 
including users and a mailman configuration, to the new box.  Basic 
strategy I have is to shut down smtp on the old server during the course 
of the migration, and once postfix is configured on the new box with the 
users and mailman aliases, switch the old box over to being a secondary mx 
for a few days while DNS settles down.

Is there a better way to do this, or some sort of online guide I can 
follow that can guide me through the process?

-Dennis Carr