The apron should wrap from the hip bone of one side to the hip bone of the other side, leaving the pleats only in back. The buckles will then be slighly just behind your hip bones on both sides. Are you wearing it this way? Otherwise, instead of having the apron wrap more on one side than the other I'd just centre it and have it a little long on both sides.

The length of the apron should be somewhere between the top of your knee and the middle of your knee, depending on your preference. Traditionally, when you kneel the edge of the kilt should be just above the floor.

In regards to sitting, I've noticed I've learned to better sit with my legs together naturally. Otherwise, a simple push on the middle of the apron will take care of hiding all your bits and pieces. When you move you may have to re-push the apron down.

Andrew.