Move your user and computer objects into an OU structure.
Keep the OU structure as simple as possible.
Do not nest OUs more than 10 layers deep.
Keep the number of OUs to a minimum.
Apply Group Policy to groups through Group Policy Filtering.
Use domain local groups to control access to resources, and use global groups to organize similar groups of users.
Use distribution groups to create e-mail distribution lists in environments with Exchange 2000.
Mail-enable security groups if separation of security and e-mail functionality is not required.
Don't simply delete and re-create groups on the fly because each group SID is unique.
Don't include users from other Mixed mode domains in a forest in universal groups.
Don't utilize local groups for permissions in a domain environment.