How can I avoid getting chain grease on my trouser leg How do you solve the problem of stained jeans? I'd prefer to avoid modifying the bike itself or rolling up the hem on my pants.

Of course, riding with short pants is out of question, now that the winter is coming :-)
Leg straps or clips (especially on the chain side) are the way to go.  Not only does this keep the pant leg clean, but it prevents the cuff from chafing or hanging up on something.

You can improvise with a large rubber band, but the Velcro straps are readily available and very easy to get on/off.  When not in use they can be wrapped around your seat post or some such.

(And if the straps -- you can use two per leg for really baggy pants -- don't do it you can buy "gaiters" that cover the entire ankle and lower shin.)