burgerdog said:
Learn to beat and heal yourself as much as you can whenever you get the upper-hand in battle and enemies retreat. Leave one up and farm some JP.
As mentioned before, spend some time as Squires and get everyone the JP Boost ability, which increases job points gained. You get JP for every single action, so yes, a good tactic is to have your party beat on each other (or a creature with a heal power like a Chocobo) for job boosting. I typically have my dudes throw rocks or do Rush attacks, something that has really low attack power; you can Focus or do a healing ability too, but sometimes those own abiliiesncan be expensive to learn.
I forget the exact values, but you shouldn't always do both a move and an action for each turn if you want to win a battle faster. If you keep your party members stationary but still attack, for example, your AT bar will fill up faster and your turn will pop up again sooner than if you did both a move and an action that turn. The AI will always do both, so if you're smart about placement, you'll generally get more turns and be able to take more actions.
Also, when enemy human characters die and leave crystals, the party member who picks it up has a choice of two benefits: a) full HP/MP refill, or b) learn one new ability at random that the dead character had. So if you encounter an enemy summoner, and your have one of your party members pick up his crystal, there's a chance he'll learn a free summon (which could be worth hundreds of JP).
There's a ton of nuances to the battle system, so it'll take a long while to really figure it out fully and find a party balance you like. Just enjoy it.