I lug my 17" MacBook Pro, clothes, pump, lunch and assorted junk each day (about 16 pounds). I used to use a Messenger bag (Spire), but the weight on one shoulder was uncomfortable, and quick starts/stops or foot down could make the bag swing to the front. I bought a Banjo Brothers bag that is a back pack with a single top-loaded space (which is basically a very waterproof big plastic sack) with fold-over enclosure with 2 shoulder straps, chest and waist straps. I keep the computer in a padded sleeve. It took a little getting used to, but I like it now.
You can see it at
http://banjobrothers.com/products/01150.php