From Lifehacker comes a recommendation that should embarrass touchpad makers (like, ahem, Synaptics):
Wear Gloves to Stop Touchpad Annoyances
Photo by Nessa Land.
Miguel tells us how he keeps his mouse from jumping on laptops:
If you're writing on a laptop, and you constantly find your palm
brushing against the touchpad, thereby making your insertion point
dance around, put on some thin knit gloves. It will keep incidental
contact from registering on the touchpad. If that makes your fingers
too thick for typing, or if you still want to be able to use the
touchpad, just cut off the fingers.
Of course most touchpads do have an algorithm built in to suppress accidental contact. Check out your touchpad settings (on Synaptics touchpads the setting is called PalmCheck). I recommend trying the maximum value. It's not a perfect solution, though. Bigger touchpads and touchpads centered to the palmrest instead of the keyboard have aggravated this problem in the last couple of years.