I like this one tip by Ed Bott on his ZDNet blog:
When nearing the 30 day mark of the trail period:
From an elevated CMD prompt: slmgr.vbs -rearm
Reboot and your are done.
One can install Vista, then setup a simple script to run at the 29 day mark to essentially extend the trial period for another 30 days without having to either reactivate, or reinstall the OS. Voila, you have up to four months to play around with it!
You can find the tip here: Vista Hands On #9: User Vista for four months, free.
Microsoft Small Business Specialists