Hit balls with both your feet together, touching each other. Try hitting 10-15 balls like this, while trying to keep your balance. Then hit 10 -15 with your feet 6-10 inches apart. Do this without worrying about how far the ball is going, just about making good contact. Then move to a normal stance. I think you'll find that your feet will begin to stay more put. You'll also begin to get the feeling of a more balanced, controlled swing. Your feft foot sliding could mean your "coming out it" so to speak.

Whenever I begin to feel like I'm overswinging or losing balance, I tend to get "happy feet." I go back to this drill and it not only helps with planting my feet and for balance, it helps with the idea of rhythem and control in a golf swing. Good luck.