Two (at least for now, I'm trying some new ideas):

I travel with a 46L mid-size duffel with back-pack like straps (Osprey Transporter 46) and also a smaller super-cheap Jansport day-pack. When my larger pack is not sitting in a hotel room (i.e. when I'm moving to a new city), I wear the duffel on my back and the smaller bag on my front. As someone said above, the smaller bag has a lot of electronics and a laptop so this results in a nice balance when walking around (arguably I look stupid walking around with a bag on my front, but it doesn't bother me). If I have to, I can cram the smaller one into the bigger one. The duffel can be carried (backpack straps are hideable) on the and the smaller bag is my personal item.

I've used this configuration for about 30 weeks of leisure travel of the last several years, 19 weeks consecutive once.

http://images.travelpod.com/users/di...g.ryan_bag.jpg