Some very good replies above! I think your original question contains the main issue:
<BLOCKQUOTE>quote:</font><HR> If I want to exert more pressure on my ski edges at the apex of a turn to prevent skidding what is the movement that will most help? Is it increasing fore/aft pressure in the front of the boot or rolling the edges more into the turn? <HR></BLOCKQUOTE>
More pressure on the ski edges will not prevent skidding, but less skidding will result in more pressure on the ski edges (all else being equal). Remember, as I have often said, we don't get pressure on our skis by pushing on them, but by allowing them to push on us. (Actually, I first heard this statement from Bill Sloatman, one of Keystone's top instructors.)
What will make your skis hold better? They must be sharp enough to cut into the snow surface. You must tip them sufficiently to create the "critical edge angle" that we have discussed in the past. You must not twist them into a skid. And you must flex and extend to absorb shock and vibration that can cause them to bounce out of their "groove" and skid. See to these things, and your skis WILL NOT skid!
And when they don't skid--when they grip tenaciously throughout the turn--you will be able to hold a tighter turn and maintain maximum speed through the turn, which will increase the "g-forces" of the turn, which will thus increase the pressure on your skis.
Increasing forward pressure on the boot shifts the pressure center forward on the ski, which can cause the tail to wash out. Forward pressure on an edged ski causes the tip to bend more tightly, while straightening out the tail. The ski bends into a non-round shape, which cannot then, of course, carve a smooth, round turn.
Rolling the edges more into the turn MAY help them hold better, but only if they were not sufficiently tipped (critical edge angle) in the first place.
Patience is really the key. Avoid harsh or sudden movements. Steer the skis gently, "tips in" to the turn, rather than pushing the tails out. Remain supple to even out the pressure on the edges. Let the skis do their thing--don't try to force them. TRUST them! And most of all--don't push on them--just let them push on you!