I've usually been able to get away with a Camera bag or a Waist pouch alongside my Backpack. Some airlines also allow a separate Laptop bag, but your mileage may vary.
I'm usually wearing my SCOTTeVEST Jacket, so I can usually carry a lot of smaller stuff in one of the 42 Pockets. (I just need to figure out a surefire way to remember which Pocket I Put something in, or I'll spend ages trying to find it.)
And Harry, BAA Finally abandoned that stupid one bag only policy last year so you are no longer forced to try and figure out how to fit a purse or camera bag inside your carry on. Now if they'll get rid of the stupid liquid restrictions as well .