A little trick for you. Take some white-out, or a silver ink Sharpie. With the bow at the rest position, make some marks on the string serving right up against the rail. That way when you draw it back, you can make sure it stays centered by keeping the marks on the string in the same position relative to the rails. If you let it get off to one side or the other, even a little bit, it'll shoot funny. And don't forget to wax those rails. 
