Over the years I've shot with Domke bags (love 'em!), the Moose Peterson WRP backpacks (great for big lenses and lots of lenses), and Tamrac bags of all sizes. However, they all scream "cameras!" and beg to be stolen. Not to mention that the larger ones tempt me to load up on gear I never use - and they end up weighing a ton.
This last time around I wanted a bag that didn't look like a camera bag and would only carry a small set up - SLR with prime, wide zoom, long zoom and flash.
I chose the National Geographic mid-sized Photo Backpack. It looks like and is a mid sized backpack - like a book bag, with one exception: the bottom half contains an area specifically designed to hold camera equipment. There's room up top for a change of clothes or a day's worth of books, snacks, etc. There's even a slot on the back for legal pads, writing equipment or a small laptop or notebook computer. You can get them practically anywhere - I got mine at B&H.
The photos of them on the net aren't very flattering, but mine is well built and very utilitarian. Exactly what I need. I have - for the moment - found my perfect bag.
Good luck on finding yours.