Everyone may already know this trick, but I thought I'd write about my own way of doing it. In Super Mario RPG, an easy way to get lots of levels is by using a star. Stars are found in boxes, but they aren't items you use from the menu, they activate on the spot. Then, for a short time, you can run straight through enemies to get experience without actually fighting them. The stars don't respawn though, so once they're gone, they're gone... or are they?
In Super Mario RPG, like Final Fantasy VI and probably some other RPGs as well, when you game over, you lose any money or items gained, but retain your experience points. You may see where this is going. Most of the blocks containing stars are very close to save blocks, so save your game regularly, and whenever you come across a star block, which should be obvious considering the rooms are usually crawling with enemies, you should completely unequip your party and save your game. Then, get the star, wipe out as many enemies as you can, then allow yourself to game over.
The two spots I found it
The second spot I used was in the underground section of Land's End. Be sure you make it through the dunes with at least 400 coins, then take off the party's gear and save. The box right below the ledge contains a star, so grab it and wipe out every enemy before jumping into the hole. In this room, crawling with lizards, you can find a wizard hiding behind some boxes to your left. He spawns in a second star box, but charges 400 coins for it. Pay up and clean house.
Once you're done, go in and out of the next room until you see and hear a cloud spawn in front of the entrance. Jump up to encounter it, and be quick, because it can run off and be hard to catch in this room. In battle, you'll see you're up against nothing, or rather, a Formless. It's immune to physical damage, but reveals itself as a Mokura if you attack with magic. You have several options here. You can continue to attack its invisible form, which will miss every time, and let it defeat you with its magic, or you can use a weak spell on it to make it reveal itself, which causes it to be vulnerable to physical damage, but it also powers up its magic. From then, you can waste turns and defend until you're defeated. I personally made it reveal itself then repeatedly attacked it, neglecting to get timed hits of course, and if I defeated it, well, that was just more EXP, but I'd eventually succumb after a few battles with it.