Developing power and learning to hit home runs is something that needs to be practiced just like anything else, if you are to become a power hitter. Just as much as spending the time to work on hitting baseballs middle the other way on a line and working on bat handling, such as hit and run or move the runner over.
In a typical round of BP (batting practice) at the professional level you end up getting about 20-25 swings. This should be true of high school and college as well. Often times there is one or two situational rounds, i.e. hit and run or 2 strike approach, followed by a couple rounds of free swings. These free swing rounds are typically instructed to be used to drive the ball gap to gap. I fully suggest that every aspiring power hitter should use these free rounds to do just that, work on driving the ball gap to gap, but over the fence. In order to hit home runs in the game you need to practice hitting them in BP. However, the trick is this, work on hitting home runs only to center field. Pick out two spots on the outfield fence slightly more narrow than true right and left center field. Every day you take BP you should play home run derby with yourself or a teammate where the only home runs that count are those that go over the fence between the targets to dead center. I choose center field as the target for my daily home run derby because it is typically the deepest part of the park, usually 400+ feet. I am challenging myself to hit for the most power possible and hit the ball the greatest distance needed for a home run.
Perhaps more important than the challenge of hitting it to the deepest part of the park is that I want to practice hitting for power while still maintaining a good mechanical swing with direction to the middle. Too often guys will sit and be visibly trying to hit home runs during BP to their pull side. This is because it is typically the easiest place to hit home runs since it is shorter. I was guilty of this approach during BP my entire life until about 10 years into my professional career. At this point I finally took a true power hitter's approach in my BP sessions. In a fairly short amount of time turned into a more disciplined "power hitter" instead of an undisciplined "power swinger". When practicing hitting balls time and time again to your pull side in BP you are typically developing a poor mechanical swing and working on your bat path coming across the zone instead of exploding through the zone. You'll notice that the flight of most home runs of the guys doing this are "hooking" towards the foul pole instead flying out straight and true. Come game time when everything speeds up, the swing path they've been working in BP turns into quick in and out of the zone or across. This leads to swings and misses, foul balls such as the one that starts fair and hooks foul on you, roll overs, poor pitch recognition, inability to drive the ball the other way, and top spin instead of back spin.
In order to consistently hit the ball over the fence in BP to center field you are forced to do 2 things. First you have to focus. Focus on the task at hand and zero in on the pitch that will allow you to drive the ball to the middle of the field consistently. You can't just swing at any pitch that's thrown near the zone and expect to hit it out to center field. This should be the same approach you take in the game in hitter's counts, i.e. 0-0, 2-0, 3-1, 3-0. You must focus in these situations and don’t expand out of your zone.
Second you must stay square with the body and front side allowing the ball to travel to the correct point of contact (roughly in line with the front foot) and stay directional to the middle of the field. The focus must be on the backside driving into and through the front side, not the front side pulling the back side around or across as well as not falling back on to the back leg. (This is what people mean when they say "back legging it".) This is typically what gets reinforced by trying to just pull home runs all the time. The front side becomes quick and pulls you off the ball, taking your energy away from the hitting zone instead of driving it through the hitting zone. By staying square you are now able to work your barrel through the hitting zone for maximum time which turns into giving yourself the most room for error with your timing. What I mean by this, is you have to have your barrel in the zone early to be behind the baseball to drive through contact, then to and through extension to deliver maximum power into the ball.
The great thing about this approach is if you are a little early, now you are pulling the ball to left field fair and keeping it straight and true instead of getting a hooking action on the ball. This is happening because you are now using your lower half the right way to pull the ball with a square front side and reacting to pull the ball by driving the backside and not pulling from the front. Now, if you are a little late your barrel is in the zone to drive the ball to right or right center field for maximum power. Although these driven balls or home runs don't count for your daily home run derby, they will count come game time.
If you take this approach daily in your BP sessions you should notice that you will be swinging for maximum effort and efficiency to develop your power while maintaining good mechanics. While you may not start off hitting ball after ball over the fence, you should stick with this BP approach and don't get suckered into the more inviting short porch down the line if you want to see the kind of in game results you'd like. Eventually, you should see yourself becoming more selective at the plate with what you do or do not swing at, especially in the hitter's counts.
It's time to stop being a "power swinger" and become a productive power HITTER!