Plug your AC adapter into your PSP.

Adjust your brightness to Level 4.

Start a UMD game or homebrew. (at which both doesn\’t matter) 

When the program launch video is being played (the one with green balls flashed and PSP text in the middle of the screen), make sure you unplug the AC adapter from the PSP when the launch sound effect is over. You will notice that the brightness didn’t fall down to level 3.

Now your PSP will be locked at level 4 – even after you suspend/resume, or turn on/off the screen.
(by holding the brightness button)

Simply pressing the brightness button will reset the level.