The SharePoint Products and Technologies governance checklist guide on the Technet SharePoint Governance Resource Center is very popular among our customers. It covers aspects from architectural planning, roles and policies, to operational practices.
To complement this official checklist, I've collected some more concrete guidelines that my customers often request when getting started with MOSS. It covers additional aspects such as driving findability and user lifecycle management. The guidelines are available here: MOSS governance guidelines.pdf
If you have any suggestions for more recommended governance practices, please leave a comment.
[UPDATE] See the updated governance guidelines here, including new SharePoint 2010 aspects.