Good water quality: Change water often to replenish minerals, prevent fish from getting sick and boost its appetite. Fish that never got sick will grow to its full potential. This practice has been implemented in discus keeping to maximize its growth and round body shape especially for contest. Do keep in mind that the water need to be aged and aerated first. Water parameter have to be similar from the aquarium.
Tank size: Bigger tank size will make fish grow faster.
Diet: Feed 2 times a day with smaller quantity in between each meals. Observe your fish aggressiveness, once it's less aggressive, stop feeding. The idea is to make your fish to eat on daily basis.
Note: this is just based from my observations.