Pilotage,
What I've done with mine is turn the brightness down to 1/4 power and I set the back light to turn off after 30 sec. Just a tap the screen to light it up again when needed. It all stays running but uses a fraction of the power while the back light is off.
Saying that, I just use mine as a 'double check' so I don't need it on all the time. If you like it on constantly I'd suggest getting an external battery which you can plug into the PDA and lasts for hours. Depending on how much you pay!! As a rough guide a friend of mine got one off eb*y for £20.
I'm all for GPS as a back up. Don't know about relying solely on one though.............