Overjumping by the horse or by the rider?
I'm going to assume you're asking about by the horse -
Overjumping is usually caused by greeness, a young horse being extra careful and giving their fences a lot of room to spare. Really, in a green horse, it's almost a good thing - treating their fences with a lot of respect makes for a careful jumper. Time and experience usually take care of it. Liking a long distance and leaving from a long spot can contribute to the overjumping. So a rider that can establish good pace and balance and find the correct distance will mitigate a lot of the tendency to overjump.
This is one of the many reasons that "green on green" is asking for trouble. Take an inexperienced horse that tends to overjump, put on a green rider that can't establish pace and balance or see a distance, and you have a horse galloping down to a fence on its forehand, leaving very long and jumping its rider right out of the tack.
Besides time and experience, carefully constructed grids and gymnastics will teach the horse to jump in an appropriate arc; if they overjump one element, their too large arc will put them in wrong to subsequent elements. This needs to be done carefully so the horse doesn't get discouraged.