Two Ways to do it...(Kinda)
Well, i use NesterJ and what I have found is that that if you are still in the EBOOT Loader screen you can simply hit start to have the PSP reboot, however if you are actually in the emulator, you can hold the power switch up (for about 7-9 seconds) and the PSP will power off.
Of course you can keep taking out the battery, but you probably have to reset date and time everytime, right? Hope this helps man
Jester: I dont know about RIN but Nester requires you to do this. However, for most cases you are right.