How do I improve my handgun aim?

I have shot around 300 rounds with my 9mm. The first day I barely hit a sheet of notebook paper. The last time I shot I hit the target almost every time if not every time but there was a large cluster about 2 inches high and right. Then most of the rest of the shots were to the right. The next target had a pretty scattered pattern but not very consistent. What can I do to improve my shot? How can I practice at home (snap caps/holding exercises?). I don't want to get out there and waste $25 worth of ammo not improving my technique. I hold the gun with both hands, keep my left eye shut, and I know how to align the sights. Body is generally squared off with the target and I'm about 10-15 yards away.