Hi,
Let me preface this by saying that there are a ton of experts on this forum that I'm sure will chime in soon and they know the backgrounds and reasonings of their suggestions, this is just what has worked for me and is not guaranteed to work for you:

1. Optimal conditions for the tree to grow: good drainage, lots of direct sunlight (unless its currently in a lot of shade, then make the change gradual), fertilizer if its currently actively growing; this will allow the tree to grow wild, then you can prune it to whatever shape you want
2. Let the soil dry until the leaves look SLIGHTLY wilted. This could be up to a week and a half indoors (for me at least, test it yourself with a water meter or a stick). Then water thoroughly. The tree may lose some leaves but it will start branching out and growing more new ones soon.
3. Pruning. If you prune back the branches, you'll lose the ability of these leaves to make more energy (so reduce water and fertilizer) but the tree should respond by sending out more branches. Pruning tends to divert the energy used by the main branches.

You can also try a combination of the above. Option 1 is probably the safest. Oh, and I read somewhere that the sunniest window is equal to a shade outside, so perhaps it could use a bit more light to reach the "optimal growing conditions". Try gradually moving it into a sunny spot.

Good luck!