Get a bigger compass.
Can you see individual degrees on your model? Few boaters can answer yes, mostly because compasses this exact are large and expensive. However, using one allows you to steer to within five degrees. And a large compass, mounted far enough forward of the helm to let the captain gaze at the horizon and the compass at the same time, is ideal-and makes for courses just as straight as the GPS.
Tab it up.
Apply the trim tabs to keep the boat level, especially on boats with deep-V hulls. If you allow your boat to lean to one side or the other, you'll constantly get pushed and pulled off course.
Set your sights.
You can't steer straight with electronics alone. You need something on the horizon to use as a focal point. No landmark or buoy in sight? Use a cloud. Just remember that it's moving, too, so about every 15 minutes you'll have to recalibrate. When the sky is clear and there's absolutely nothing to focus on, use wave direction. It may be tough, but you can do it. Just imagine I'm there, leaning over your shoulder, ready to scream my head off as soon as you drift more than two degrees off course.