All the responses here are keys to improving your sharp angle crosscourt. However, there is more to hitting it at higher levels.
The first key to a significant crosscourt topspin is to hit more outside the ball as you brush up. This gives you the correct angle of the racquet plane as well as emphasises the spin that will help carve out more angle as the ball is moving in the air.
To do this, while hitting more out in front is the right idea, you must be careful: Hitting too far in front creates the impression to the body that the ball will be hit well wide. In addition, hitting too far in front, the racquet will tend to flatten out instead of hitting more up the outside portion of the ball because hitting more out in front makes the player usually open the hips and shoulder plane too early.
Hit staying very sideways, closing your stance more than neutral. (Open stance opens up the hips even more and it defeats the necessary posistion to hit up the outside of the ball.) Staying sideways will allow you to emphasize hitting up the outside of the ball without pulling the ball too wide.
Keep your dominant elbow closer to your side through contact. This helps keep you from "steering" the ball and raising your elbow (which makes you 'push' the ball instead of stroke it.) Let your non-dominant arm and hand drive up around this dominant's elbow (which acts as a fultcrum of a 2nd class lever at this point).
If you stay sideways long enough, you will hit out around the ball with significant spin and racquet head speed to create the severe topspin crosscourt you are looking for.
Try it and see if you don't get what your looking for.