Originally Posted by Flying Mullet
You have to have a job that has more than one screen of main abilities, like the priest or chemist. Pick an ability, then when the comfirmation pops up, hold the square button and press down. The cursor should move one screen down. Make sure that the cursor is on an ability that you don't have enough points for and is grayed out, then click on confirm. Usually your job points will go to zero, but exit out of that character's ability screen, then go back in and you should have 9999 job points. One word of caution, some choices of grayed out abilities will either take away a small amount of JP or truly zero them out altogether. I don't know which ones they are, but if you check on gamefaqs I know that I saw a table in one of the faqs that lists which abilities to use to get the glitch and which to avoid.