During random battles (not story-driven battles), earning experience and JP is necessary for the growth of your characters. Since enemies in random battles can almost always be handled rather easily by your party (as they grow with the strength of your party), you can take out all but one member of the enemy party rather easily. Leaving that enemy alive, you should then have your characters attack one another, healing with Chemist skills and White Magic if you can, until you can no longer survive without killing off your party. After that, go after the last enemy and eliminate it to end the battle. This way, you can create experience and JP you wouldn't have gotten in the battle otherwise, lessening the amount of random battles you'll have to fight to build your characters' levels.
If you have a Chocobo in your party youll definitely have Choco Cure, a useful skill that can be used over and over again. Choco Cure restores some of the Chocobo's HP, as well as characters in front of it, behind it, and to the left and right of it. When trying to do the "friendly fire" experience trick, having a Chocobo in your party to heal characters grouped together each round allows you to survive the fight even longer, building experience and JP levels even higher.