I have only seen a few threads on how the floor mats slide all over in our cars after a little bit of use and have a cheap reliable solution to this problem.

You'll need:

4 2x4" self stick industrial strength velcro strips, per floor mat. I got mine a Walmart.
1 tube of seal all
Something heavy to put on the velcro strips while they dry, I used a dumbbell.

On the driver/passenger side put 2 strips on the bottom, there is a smooth line that goes across the floor mat on the bottom side, closest to the seat. Put one velcro strip on each side. Liberally apply seal all to velcro strips and add a weight on top of them while they dry. Sand the spikes down flat on the top side of the floor mat that would be under the pedals and add seal all to 2 more velcro strips. You only need to get rid of enough spikes to make a flat surface to adhere the velcro strips to. Add floor mat to car once seal all is dry.

I used the course velcro and didn't need to use the "fluffier" velcro pieces, as the course velcro stuck to the factory carpet. The floor mats do not move at all during operation of the clutch, brake, or gas pedals.

I hope this helps, I went without a driver's side floor mat for a while and now it stays in place. This won't ruin the carpet or anything else in the car.