Alan, if you use the clamps as described below, you will only experience minimal wear on them.
When I use them, I use the following technique. Let's start with the knife handle to the left..
I hold the knife handle with a relaxed grip in my right hand.
I rest my heel of left hand on the clamp, with my finger tips resting on the blade.
I use my left hand to guide the blade down the stone, and use my right hand to keep the blade perpendicular to the direction of travel.
I apply downward pressure on the blade with my fingertips. I try not to apply pressure directly on the clamp. (This puts the force where you want it - on the blade and not the clamp.)
To do the reverse side of the edge, instead of pulling the blade towards me, I flip the knife over and place the handle in my left hand and use my right hand to hold the clamp and blade. The technique from here is the same as before, except for the fact that you have switched hands.
The clamp does slide down the stone, but I try not to exert any more pressure than necessary to it directly. The clamps are heavy duty metal, so they will not wear down very quickly. With the technique that I described, I expect that they'll last forever (or at least through my lifetime anyway).