I usually keep mine hung from a skirt-hanger in a garment bag, hanging in the back of the car when I travel. If you can't do that, then folding it, and rolling it, should work.

Close the kilt, as if you were wearing it (that is, the aprons are folded shut). Now, fold the kilt in half again (fold on the vertical), with the pleats on the outside.

Now, if you are putting it in a suitcase or something rigid, you can just fold this in half (on the horizontal) and lay it flat in the suitcase, and you shouldn't have a problem. When we ship kilts for rentals, we ship them this way in boxes.

If you are packing it in a duffle bag or something soft-sided, rolling it, rather than folding it, would help keep the pleats neat.

They do make carriers just for kilts. I've seen some sell for well over $150. Now, I, personally, am not going to spend that much for a garment bag, no matter how nice it is.

We've found one for considerably less, here:
http://giftshop.scottishtartans.org/outfitter.htm

It's like a garment bag, but made of a heavier duty fabric. After you hang your kilt in it, there are straps that you secure to keep your kilt (and jacket, etc) in place. Then you can fold the whole thing in half and carry like a peice of luggage. Really quite nice. If I traveled more often this is the one I would buy for myself.

Aye,
Matt