SH - Unfortunately once they tie up they can be prone to it from then on. You need to watch what you feed them (consult the vet about a specific diet plan if possible), and always make sure you warm up and cool the horse down very thoroughly before putting it away. As I said earlier, tying up is a build up of lactic acid in the muscles around the hind end and over the back, the the longer you can walk the horse out after a ride or lunge, the better. Keeping the horse in a paddock for as long as possible each day will also be helpful.
Gatorade is ok, but I would hesitate to give it as a regular source of electrolytes because of the enormous sugar content. Just go and buy a tub of equine electrolytes. You can get it as a liquid or a powder/granules, this will probably work out cheaper than trying to give your horse a bottle of gatorade each time it works hard.
I give my horses electrolytes each day over summer as a maintenance dose, because it gets so hot here. If I've worked them hard, they'll get a double dose.