Photo Study of the Roger Federer Forehand

Roger gives us a good illustration here of how to slide into a forehand on clay. Note how the front foot is angled across the direction of the slide, and the back foot is tilted onto its inner edge. These foot positions offer the most twist-resistance for the ankle if the player should hit some kind of bump or slow spot on the court while sliding.

Unless you're sliding toward the net, which is relatively rare, you'll usually need to hit with an open stance as Roger is here.