Dynamic: Does the seat tilt backward enough? If not: adjust the weight knob at the front, under the seat (loosen, shift to to a higher level, and tighten again).

The CorrectSit and the ErgoSolex have a seat tilt that can be adjusted with the handle on the left side of the seat. Set the seat to a more backward tilting angle. (Note: the Fixed-Tilt models have no manually adjustable tilt). 

The ComfortMove has an automatic built-in seat tilt. This automatic tilt only works if you are properly seated deep in in the seat. If you sit on the tip of the chair, or even half way in the middle of the seat, the seat actually tilt forward, and you may slide off. If seated all the way in the back of the seat, the seat tilt works properly and you won;t slide off - instead: you will enjoy proper posture support that the chair is designed and famous for.

Other reasons:

- Slippery clothing, like silk or nylon, can cause sliding as well. 

- Worn upholstery due to the wrong cleaning detergents that contain alcohol. In that case, the seat needs to be replaced.