I was told that, in the beginning, you should use some type of gate/barrier that makes them go all the way out to where they have to turn.
Another thing I've heard is to always keep them jumping in the center of the jump
I have had good luck with just a yard stick. If the dog is not too bad about it, often just laying the yard stick on the broad jump will help them jump straight. Sometimes the handler has to held the stick so that it forms an angle between the handler and the jump.
I use a bar jump set low abut 16 inches across the middle of the broad jump so they get the visual of jumping up and straight over the broad jump, while I call them straight over the jump, and gradually move to the side, once they are steady in that I do the broad jump exercise with a dial rod stuck in the end of the last broad jump board. so it helps them jump straight out, then turn around it. I also set one of my ring gates to the side, so once they jump they have the gate as a guide straight into me to help with the front. Even after they have the jump down well, I'll still set the whole thing up again as a reminder now and then of how to jump it so they don't develope a habit of cutting that corner off.