I don't know if there's a gauranteed way to get your coral to grow bushier, rather than elongated.
I do have one observation that may be of help...
I keep a few different kinds of Acropora frags growing. One of them was placed horizontally on the live rock near the front of the tank. That one kept growing longer, and didn't branch. Finally it got too near the glass, and one day while I had my hand in there cleaning, I knocked the coralite right off the growing tip.
Since then the coral started branching like crazy.
I have accidentally done the same with a Montipora digitata, but it did not branch out in response.
So I don't know whether the branching in my Acro is just an aberation, or maybe a specific reaction in this particular species.
But it may be worth experimentation.