Just the act of rolling up your pant leg one or two times usually covers the problem of it getting caught in the gears.
I've also resorted to tucking it into my socks. It seems silly to buy clamps and devices when these two "free" options are available.
Nice that it's enough of an issue that it's being brought up online. When I started mountain biking, some of the magazines and online sites basically accused a person of being a "dork" if they didn't dress in the appropriate gear.
I don't want to wear tight padded pants just to ride my bike. I've been wearing a t-shirt and jeans for riding for about 4 years now. I wear them in the summer too. I figure my legs will be more protected should I fall, wearing canvas than they'd ever be in a goofy, padded pair of semi-shorts.