I put painter's tape down so I'm guiding it straight to start with, before I get to the foot, and also learned to use my non-dominant foot on the pedal recently. I don't know if that is helping me keep it straight but it sure did help give my back and neck some relief. I think it straightens my spine up.
So ... I'm right-handed and use my left foot on the pedal.
I also use a stylus, the purple thang, to make sure the foot doesn't push the fabric to the left at the end of the seam when I can't see it well.
I am going to use a pad like Jan suggests for a more firm "bumper" though.