We've actually covered this topic in at least two other threads, but here goes again.

I teach this to my new kilt wearers. Girls used to learn it very young, and women have used the "sweep the pleats or scoot the 'boot" drill for many years.
Watch old movies and pay attention to how women used to sit down

"Sweep the pleats" - when sitting, reach one hand under your butt and sweep the pleats down in a smooth motion towards your knees as you sit. In a car without Jeff's pivoting seats,turn sideways, keep the knees together, and sweep, sit, then pull your legs in.

"Scoot the 'boot" - Back-up to a chair till about a foot away. Squat down keeping yur back upright and sit on just the edge of the chair, knees toghter. Then slide your butt back into the seat of the chair.

Both methods work fine to keep your pleats smooth. Both methods maintain modesty.

Pratice until they become so natural that you don't even think about it any more.