My Weathertech front floor liners fit great for about a year, but then the driver's side one seemed to kind of shrink. It no longer fit closely along the edges and began to constantly slide up toward the pedals, requiring frequent readjustment.

So I put a piece of slightly "sticky" rug pad between the mat and the floor. It is holding great so far and allowing no slippage whatsoever, although the big gaps along the sides are still there. Still, a great improvement.

I used a piece of Teebaud Nonskid Rug Underlay from Horchow.com. These are really good rug pads that I use in my home as well. Hope this helps someone as the slippage is super annoying!

Rug Pads