If you want to hit the ball further you need to have lower body power. After all the glutes, hamstring, adductors, quads etc that make up the hip musculature are capable of producing the most power of any muscles in the body. Research testing competitive long drive athletes has found the average vertical jump to be over 30 inches. 8 inches higher than the average PGA tour player, and comparable with guys in the NBA!
So how do you develop this type of explosive power?
Mobility, mobility, mobility. Mobility is the first step in developing power in any part of the body. Flexibility helps muscles create elastic energy, through the stretch shortening cycle. Additionally, the closer a muscle is to its end range of motion the less force it can generate when it contracts. Ergo, the more range of motion you have the greater force the muscle can contract with from the same position.
The feet are you only connection to the ground. Good ankle mobility is paramount to start the power sequence, focus on ankle dorsiflexion and eversion. Second attend to the hips, internal and external rotation, and hip extension. Lastly, pelvic mobility is essential in order to use the lower body appropriately in the golf swing, we must be able to separate the lower body from the upper body.
Strength. Strength is the basis of all athletic improvement. Get stronger and your musculature endurance, size, strength and power output will improve along with it. Pick a solid strength training protocol and apply it to compound exercises that work the lower body musculature together in sequence (squats and deadlifts would be the correct answer).
Speed and power. After teaching the muscles to contract with maximal force we must teach them to contract quickly. This is where the fun starts. Think fast when executing these moves. Speed squats, dumbbell snatch, kettlebell swings, squat jumps and lateral jumps are some of my favourites.