So you've got that job you've always wanted, but no real way to get the skills you want. Most enemies only give 1 JP when they die and they die so fast... what's a person to do? Clevel, of course. Just remember to clear your exp debt first.

Early on (about 1-10), Tackle is a good way of gaining jp. Sub Fighter onto whatever class you want and tackle things into oblivion. Try to find mobs that don't hit you hard, but have lots of hp. Green Ogres in Relidia are a prime example of this. Cedus/Mortis will tell you that they're a great way to gain experience as well, but I find them too slow, so Asps for experience, Green Ogres for JP.

Later (about 15-whenever), you can kill mobs in Imperial for decent JP. The exp is fairly good depending on your level, too. Black Dragons are very weak to white magic (Sanctify works well) and if you have Sensei and decent willpower (not wholly necessary), you can kill them in one hit with Aurabolt.

As part of my fixing of this site, I'll also add that if you can kill them quickly enough, the mobs found in Darryl's Tomb are very, very good JP. Bard's Crypt as well to a lesser extent, less JP there but the mobs are easier.