Do not try to break the board, just strike at a point in space behind the board. The board will just be something you must go through to reach the target point. If you punch at the surface of the board, you will hit it, and probably stop there. It is a mental thing; if you see the board a barrier you must go through, you tend to concentrate on the board. Instead, concentrate on your relaxation, speed, technique, striking angle, and the target point in space just behind the board; the board itself is of no concern. It could just as well be a sheet of paper. One thing off-road motorcycle racers learn quickly is that, if you look at an obstacle in front of you, you will hit it. You have to look where you want to go to avoid the object, not at the object. Do not look at the board, instead, visualize your target as the point in space behind the board.
The only difference between a technique used in training and the one used in breaking is that, in breaking, a board is between you and your target. People tend to train one way in class and then when it comes to performing a break, they think all they have to do is hit harder. In training, a perfect strike will be a thing of beauty that will make spectators gasp. In breaking, a perfect strike will go though the boards like a hot knife through butter.