Depends on conditions. If it's over 60*F, I wear shorts (either bike or baggy, whatever). Down to about 45*F I wear running tights ($8 at Target). Below that I wear lined running pants. If I'm running just a mile or so to grab a sub or something I just clip up my jeans.

Use a velcro or metal clip to bind your pants so it doesn't touch the chainring. If it's touching the chain ring there's a chance it'll catch in it. I have a couple pair of jeans with tears in the right cuff.