Personally, I don't do any horseback riding in the kilt, Mel Gibson notwithstanding.

For driving, there are several techniques, you just find the one that suits you and your vehicle.

Sit sideways, smoothing the pleats with the hand(s). Turn, raising your weight a bit & smoothing as you go

Raise up and smooth once you're facing front. This is probably the most common ploy.

Some like to sit on a towel, then hold it at their sides as they turn---works pretty well.

Some just don't worry about it. The wool & PV hold a crease like nothing else.