put a jab in opponents forehead and throw your cross underneath your jab........ I.E: aim your cross at the underside of your jab glove (at the laces/ strap). Work's even better when you're opponents wearing headgear.
Honestly, the easiest way to fix this is to keep your chin down. In short, what you should probably be more concerned with is the fact that when your hitting the forehead. Your chin's likely up in the air. Almost all beginner or novice strikers look at where they are going to punch and if you're looking at an opponents forehead, your chin's probably up in the air. If you can imagine looking through your eyebrows at your target, you'll find your chin stays tucked and you'll be less likely to aim up high at the forehead. Imagine a square that goes across your opponents shoulders, through the collarbone and down to his waist, that's where you want to be looking and you'll find your punches won't float up high on the forehead nearly as often (unless the opponent drops down into it).