I have several MVT's.
First of all.... backup backup backup. I rely heavily (more than I like to admit) on my PPC for all aspects of my professional and personal life, and regular backups have saved me several times.
Second, export settings for any software that has that option. Restoring a backup is great, but if you can import settings after that it saves a lot of headaches.
Third, install a better file explorer (I love Resco), and some software to help optimize your device (MemMaid is my favorite). These two apps really help keep a PPC running efficiently.
Last, a tip I picked up somewhere online a few years ago. To set the storage memory and program memory add up the total amount free in both categories, divide that total by 6 and set the storage memory slightly above that number. This frees up the most memory for program applications without the device resetting to an even balance without you knowing it. (don't know if this applies to WM5 and later)
I would also second Jon's advice to get to know how to handle the worst when you have time. That saved my butt when my device crashed while I was at a conference recently; everything was backed up on my laptop and I was up and running again in no time.