How to migrate a domain from one forest to another forest.
Usually you use the ADMT for restructuring an Active Directory environment.
Planning is the key as with anything but you've asked what the best practice would be and to answer that what I'll say one must understand core concepts of domains, forests and trees in Active Directory, how Active Directory works, how DNS services work because Active Directory relies upon DNS to function,, along with trust relationships between domains, trees and forests and Active Directory partitioning, and restructuring before one attempts at migration or restructuring.
For that basic knowledge set I'll suggest reading the following MS KB article on Forests, domains and trees in a MS Active Directory Environment. http://support.microsoft.com/kb/310996/en-us
I recommend you also read up on the ADMT for restructuring an Active Directory environment.
here's a link on downloading the ADMT which provides a brief intro of what it's used for. http://www.microsoft.com/downloads/details.aspx?
I'll also suggest going to Tech Net for white papers on most things Active Directory.