Okay, it's been a couple of days now that I've followed the advice on this thread (i.e. not powering down my N800). Instead I set mode to Offline and then Lock screen & touchpad.

Before (with frequent power-down & reboots) I was charging the unit twice a day.

Now, leaving the N800 in a stand-by state, the battery is lasting for days!

In the HOW-TO guide, this power philosophy should be the very first item for new users.