Do you know why you're off? If you know what your specific issue is, let us know and we can offer specific suggestions.
If you're not sure how/when/why you're going wonky, it's time to get a friend to watch you cut OR slow way down, look at everything you're doing, and measure after every single cut and try to figure out where exactly you're going wrong. If your cuts come out perfectly doing that, it could be just that you're going too fast and missing it when you're a little off. (Been there, done that!)