Of all the baseball hitting drills I have tried over the years, here are two which I use over and over.   The Self Flip drill is for more advanced players and the soft toss is for younger players.

Self Flip
The self flip drill is very valuable for advanced hitters. It will force hitters to develop quick hands and strong forearms.

To perform this drill the hitter will hold the ball with their top hand as their lower hand grips the bat. The hitter flips the ball up no higher than eye level into the hitting zone. At this time, the hitter will grab the bat with both hands and hit the ball.

Obviously, the goal is to hit line drives and in the direction of where the ball was flipped, (middle, inside or outside). Hitters will notice that very quick hands are necessary to hit the ball consistently solid.

Soft Toss
My players tend to use a lot better form when we are hitting soft toss than when they are hitting live pitching, so before each game instead of going to the cage to warm up, I throw about 25 balls soft toss (whammy) to each of them to get there swings ready for the game.   They seem to have better cuts during the game with this.