I've used a strap around the bottom of my jeans, but I really didn't like it. In additional to the small additional hassle, I found that it made biking less pleasant. It made my leg hotter and sweatier (because you've closed up the opening at the bottom of the leg), and it made that leg on my pants more constricting.

The absolute best solution I know of, if you can get it, is a chainring guard. It's just an additional (slightly wider) plastic ring on outside of your front rings. Here's a picture of one, though usually they're a little more discrete:


The main issue is that, even though they've been around since the 70's, if your bike didn't come with one they're next to impossible to get added after the fact without paying a bunch of money (a fact I discovered when I asked to add one to my commuter road bike). Many shops said they might be able to do it, but it would require replacing the entire front...gear thingy (sorry, blanking on the term right now).

This is the only one I could find that looked like it might fit on a 50 tooth front chainring:
http://calhouncycle.com/ProductCart/...idproduct=1242

It seemed like it was possible to find one at a reasonable cost for a 48 tooth front chainring, here's some from Salsa:
http://www.salsacycles.com/chainrings.html

Hmm, I guess Velo Orange is now selling one that's supposed to be big enough for a 50 tooth chainring, don't think it was available when I last looked (Sugino Chainring Guard):
http://www.velo-orange.com/suchgu.html

I think the chainring guard is the best solution - it looks like it would quite do it, but I've been riding with mine, commuting to work in jeans, and just haven't had any problems at all. No rips, no snags, haven't noticed any oil on my pant leg - nothin.

Here's a larger chain guard - the SKS chainboard chainguard:
http://blog.thepracticalcyclist.org/...hainguard.html

Last time I checked it only did up to a 48 tooth front ring, but it could have changed, or perhaps it's ok for you.