This power-saving tip is pretty much for the DS guys only, sorry PSP folks.
If you check out the bottom of your Nintendo DS, you'll see another slot. This is where stuff like the Rumble Pak or the Guitar Hero add-on goes. You can also toss in your older Game Boy Advance games and play them on your newfangled system. The benefit here is that when you're playing your GBA games, only one of the screens is being used, saving you precious wattage.
The only problem is that Game Boy Advance games are getting hard to find nowadays. So unless you've stockpiled a few (*cough*), you might not have a whole lot (or anything) in your current library to cram in there.
That's about the time you're going to want to visit your local video game resale shop.
Which we'll discuss another day.