Did you know that you CAN play those newer games? Like Persona PSP, Gran Turismo, Disgaea 2, and practically every new game that needs 5.55 or even 6.00 FW? Here is a tutorial by me. Revised version.
1. Download Yoshihiro's Decrypter (http://www.pspgen.com/game-decripter-by-yoshihiro-pack-rtu-telechargement-190007.html, you need to be registered to download).
2. Unrar it.
3. Copy the files in the PSP/GAME/ folder.
4. Pick a ISO file.
5. Download UMDGEN (http://www.pspgen.com/divers/telecharger-145702.html).
6. Open your ISO with UMDGEN and browse to PSP_GAME/SYSDIR.
7. Right click EBOOT.BIN.
8. Extract it somewhere.
9. Copy it to the root of your memory stick.
10. Unplug the PSP and load Yoshihiro's Decrypter.
11. Press X and wait a few seconds.
12. When the PSP has restarted, re-plug the USB cable and connect it to your PC.
13. In your memory stick root, there should be a new folder called "DECRYPTOR", and inside of it there should be a file called EBOOT.BIN. Take it and copy it somewhere safe. Do not mix it with the EBOOT.BIN you previously extracted from the ISO.
14. Open UMDGEN again, load again your ISO, go to File->Filelist->Export, type "Filelist", and save it somewhere.
15. Go to PSP_GAME/SYSDIR, right-click EBOOT.BIN and select delete.
16. Right-click somewhere in the SYSDIR folder and choose Add -> Existing File(s)..., and browse to the EBOOT.BIN you found in the DECRYPTOR folder.
17. Now go again to File->Filelist, and this time select Import. Choose the Filelist you previously exported, and click "Yes".
18. Click the "Save" button in the toolbar, select "Uncompressed (*.iso)", name it and wait a few minutes.
19. Copy the newly-created .iso file into the ISO folder of your PSP.
20. Unplug and start it. It should work. If it doesn't, try updating your custom firmware (many games work only with 5.50 GEN-B2) and/or changing drivers (this can be done by pressing select and by scrolling to the "UMD ISO MODE" voice. If the menu doesn't show up, power off your PSP, keep the R trigger pressed and power it on again. A recovery menu should appear. Go to Configuration, and look for the voice "Use VshMenu". Change its parameter to "VSHMENU", then go back and reboot the PSP. Now the menu should appear. Try changing the drivers, it may work. If it doesn't, just pray that someone smarter than us comes up with a solution).
Thanks AnalogMan for steps 14-17.
EDIT: Nevermind. I am stoopid and it appears I can't read properly. Forget it.