Swing video would certainly help specify what you should do. Here are some options though:
1) The Wall Drill -- simple. Put your head against a wall and make slow motion swings without a club to feel your head location.
2) Tilt your neck at address so you are looking more towards your left (front) foot. My instructor had me do this because I was tilting too far to the left too. Its basically an anti-Jack Nicklaus neck tilt. Tilting "Jack Nicklaus" style helps promote left tilting on the backswing. Tilting the neck the other way, helps promote less leftward tilt.
3) Feel the shoulder turn moving more "back" or to the center of your stance, with your front knee kicking in just slightly also towards the center of your stance. Helps if you're over-doing the Stack and Tilt backswing stuff.
4) You may also benefit from adding more shoulder tilt at address, particularly with driver (left shoulder higher than the right shoulder) to also promote less leftward tilt.
Too much leftward tilt on the backswing is simply caused my not extending the spine enough, so if you can feel more extension, that would help.