Fully understand the structure of Active Directory before designing.
Secure any external namespace chosen by registering it so that it cannot be used anywhere on the Internet.
Start a domain design by considering the single domain model first.
Consider using multiple domains for specific reasons only.
Consider using the federated forest design model when uniting two disparate Active Directory structures.
Control and optimize replication traffic by using sites.
Upgrade any down-level clients to reduce administration and maintenance.
Use domain rename sparingly, and only when faced with no other alternative.