Design domains sparingly: Don't necessarily set up multiple domains for different remote offices or sites.
Purchase any external domain namespaces that theoretically could be bought and used on the Internet.
Strongly consider using Dynamic DNS in an AD environment.
Consider using cross-forest transitive trusts between two disparate Active Directory forests when merging the forests is not an option.
Place the infrastructure master role on a domain controller that isn't also a global catalog unless all domain controllers in the domain are global catalog servers or you are in a single domain environment.
Use the ntdsutil command-line utility to transfer OM roles in disaster recovery situations.
Use global groups to contain users in the domain in which they exist but also to grant access to resources in other trusted domains.
Use universal groups to contain users from any domain in the forest and to grant access to any resource in the forest.