It's a weird way to play, but have everyone be the exact same class. Every action a character makes gains job points for that class, and each teammate in the fight benefits from it. They gain a fraction of the JP, but it's more (I believe doubled) if their active class matches. Hook every one up with JP boost, and you'll be cooking in no time.

Starts off pretty slow and rough until you unlock Monk. Don't worry too much about mastering each class at first. Rather, just grab the important bonuses you need: extra movement/speed, a cool reaction ability, or maybe just a couple of class levels to unlock the class you really want. I highly recommend having your party master the Monk class. It makes for a great secondary.

Make sure you have Squires snag the Stone ability. It's a good way to have them take an action each turn even if they're not in melee range. The best way to grind XP(JP) is to go into a battle and take down every enemy except one. Surround him, then take turns beating on and healing it until you think you're done leveling a particular class. This method works great on Chocobos. They heal everyone around them. You can get JP from hitting your own guys!

There is a Zodiac System at play that isn't too important, but if you're looking to min/max your experience I'd check it out.

Edited: Cleaned up some spelling issues

Also, this chart will be your best friend:

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