lots of good suggestions above....sometimes it is caused by the needle flexing as you move the fabric...so make sure you are going a consistance speed...actually as you learn and go faster...you may get less flexing as the needle stays in the fabric for a shorter period of time...using a faster machine speed may also help...(eg...keeping the stitch length shorter). And using poly thread with the right sized needle also helps....most thread manufacturers will have some sort of a chart on their websites for suggestions for needle size and thread size for quilting....