I use the Junon trick every time I play through.
When you're in Junon for the first time, theres a big machine thing that goes up and down? (I really don't know how to explain that thing) and you run underneath it, coming from the shops. You then come into a corridor. In disc 2, when you reach this corridor, you go right down the long stretch (there are shinra soldiers doing a routine in it at this point) to get to the underwater reactor. However at this point in the game (disc 1) you're just supposed to run right through and perform the routine for Rufus before getting on the Cargo Ship.
But if you run down the corridor, you'll see a red box on the wall next to the shinra soldiers. Press [OK] at the red box and an alarm will go off and the screen will start flashing red. Then if you run about in this corridor (and ONLY this corridor) you can get into fights with enemies you're not supposed to fight until much later in the game, and they all give high EXP (for this point in the game). Its a really easy way to over-level your characters early on. My process was to get Aqualung and give E.Skill to Aerith, that way she was killing all the enemies and thus getting her higher limit breaks.
As the enemies also give disc 2 gil, I used to fight until my characters had run out of mp, go to the junon inn, sleep, save, and come back and fight more. I only left when Aerith's limits were as high as possible.
And by the way - its better to do this when you can come back to Junon and you're slightly more levelled (when you get the buggy) because that allows you to get Aqualung from the desert and kill the enemies quicker.
Sorry for the long-winded explanation.
