Choosing an RDN for new objectClasses in LDAP?

What are the best practices for choosing an RDN when creating new objectClasses in LDAP? I know you can choose from uid, cn, ou, and so on, but what practices should one follow?

The RDN is essentially (part of) your primary key. The same principles apply:

• Something unique about the entry (or a combination of things).
• Something that doesn't change (or doesn't change very often).