I usually bring a tripod type of camp stool along so that I don't have to sit in mud or dust around camp. I have something called a PackSeat which I bought at REI. It probably is not the lightest available, but it is sturdy. I was able to cut about an inch off of each leg, grind off the burs and put the plastic end piece back on, thus saving a few grams.
I have taken on some strenuous week long trips and have had to kick non-chair bringers out of my chair.

Edit/update: In looking at the REI website it looks like they no longer have the PackSeat, but they have their own branded "Trail Stool" <http://www.rei.com/product/765283> which is similar, but looks improved. It weighs in at 18 oz.