Put a piece of tape farther to the front of the pressure foot so the fabric doesn't go into the foot at an angle.
Also, look to see how you are sitting in relation to the direction the piece is going into the machine. If you are sitting at an angle, there is a tendeny for the fabric to also be going under the presure foot at an angle. If you are sewing a long piece....stop and readjust. The suggestions above are good but the tape down front helps as well.