I've found Wald folding baskets to work great on my commuter. They're out of the way when I don't need them and the attachments are so that I don't have to worry about anyone lifting them off the bike when I'm not around. When I commute, I just drop my bag in the basket and go. No tie downs, no straps, no sweaty back, no worries.
http://www.waldsports.com/index.cfm/...ingbasket.html

Another alternative I've found works for me is the bicycle business pannier. It attaches like a pannier, is easily removable and large enough to carry any clothes I may need and keep them wrinkle free when packed correctly.
http://inertiadesigns.com/catalog/pr...roducts_id=168