I have some general remarks for you, and some specific ones.
Control arises from optimized, repeatable mechanics (that lead to a highly consistent release point).
IMO, you should spend as much time as you need optimizing your mechanics first, and control will very naturally follow.
Here are a couple of other 'tricks' that you can think about for making the most of your pitching mechanics, at whatever stage of refinement they are currently at:
(1) Wherever the catcher has set up, throw to a "dime" in his glove. I.e., don't be satisfied by the concept of throwing to the strike zone, or the catcher's glove--imagine a dime-sized spot inside the catcher's glove and have as your goal to hit that dime on every pitch. (Does that mean you should beat yourself up when you fail to achieve this goal on every pitch? No. Setting high goals and stretching yourself to achieve them is much more about the "process" than any single "result".)
(2) Keep your eyes focused on your target (the "dime", see #1, above) from before your first motion to the plate until you have released the ball. Not every elite pitcher does this, but the ones who don't look strange to us because they are exceptions. Do not underestimate the power of your eyes to provide quality feedback for your brain which, in turn, must pilot your body through a high quality pitch delivery. Our "predator configured" eyes--that is, both eyes located on the front of our face, for successful targeting and attacking of prey--are extremely powerful tools when locked on to a target that our brain is interested in.
All of that being said, you can imagine just the sort of drills pitchers have done since time began to develop control. Basically, they all boil down to drawing, constructing, buying, or whatever,... some kind of target to throw at. And then, repetitive throwing at the target.