Watch the RH Drills #2 Play-along online guitar lesson by Cathy Fink from Clawhammer Banjo: Beginner Module 1
This drill will train your right hand to hit the string you WANT it to hit! In this case, intention is 9/10 of the law. Don't worry if you hit 2 strings, or the wrong one. In time, you'll gain the accuracy you need.