At customer events a few years ago, we started handing out one-page "Survival Guides" (printed both sides and laminated) with links to important resources. The wiki versions of the Survival Guides are meant to do the same things without killing trees or using laminating machines. Please add useful resources you know about to this list - it is the wiki way!
Check your licensing. Avoid VM activation and legal issues. For example, when you do a P2V of Windows Server 2003, ensure that it is licensed with Volume Licensing (if you use it) and not OEM install.
Ensure that your physical machines are patched up to current requirements before conversion to a VM.
Carsten Siemens edited Revision 10. Comment: Fixed misspelling