I try to raise my party evenly. I have a group of three, and once they level up, I switch to my other group of three.

Early in the game, I stayed in whatever area I was in as long as I could leveling up. I just wandered around areas that were open to me, killing monsters.

As more areas and more hunts opened up, I started wandering, chasing down the hunts and the optional side quests. I never used chocobos or teleport stones, in order to gain the loot and experience.

Currently, my party is 68th level. I've done every hunt available to me except the Behemoth King (haven't had time to make it appear) and I've only done half of the fight against Gilgamesh. My next location will be the "tower on the distant shore" (I forget the name right now). Just not liking that my experience is a little less due to Reddas being in the party.

One other thing that is helpful is equipping the entire party with the item that doubles experience. (I'm bad with names)