I just go with mtb shorts. If I know I'm going to be off the bike for a while when I get there I'll toss some boxers in my bag and take the liners off when I get there. Yeah it's a pain but not nearly as much as the time I sat on a nut after standing for a speed bump hop.

If I really want to be wearing regular pants I just do the same thing and wear the liners under some jeans. I HATE riding in jeans though, they feel so heavy and drag on your knees. Anything longer than knee length that doesn't have an articulated knee is such a pain to ride in. If have some great Cutter knickers for slightly chilly days. They're super dorktastic but oh so comfortable.

Some times with a long ride I'll wear bib shorts and a jersey then throw some regular shorts and a t-shirt in a bag. I'll be totally set if it ever becomes stylish to walk around with bib short straps hanging out of the shorts I pulled on after the ride. I've done that enough times.