IIRC you can also charge the PSP through the USB port so you can get one of those newfangled portable battery/usb charger combo's for longer trips.
(Or, obviously, just a USB cable if you have access to a laptop/pc)
Not that this makes up for the fact that PSP battery life can be a tad short for some things. Although, seriously, four-five hours of continuous use is really plenty for most likely uses of the device. (Yes it's short and yes you get a 'ok, it needs charging... again!' feeling from time to time but it's not as earth shattering as some people make it out to be)