OK, lots of mis-information in this thread. Let's have a seasoned traveler (me) set some things straight.
1) The security people don't care how "poofy" the backpack is, as long as it fits through the scanning machine.
2) If you wear the sweater and it's obvious that it's over other clothing, they may require you to remove it before coming through the metal detector. (You are not allowed to go through the detector while wearing a jacket, some personnel will consider a sweater to be equivalent to a jacket.)
3) Extra shoes can be left alone in the backpack. The only shoes you have to worry about are the ones you are wearing.
4) You will have to remove the laptop from your backpack and place it in a bin with nothing else, but I've never been asked to turn my laptop on or otherwise demonstrate functionality. Not even when I was pulled aside for a random search and had them go through all of my luggage by hand.
5) As long as the food is not liquid, you're fine. I bring snacks in my bag all the time (swedish fish, chocolate, sandwiches, etc).
6) Remember that any liquids or gels (toothpaste, deodorant, etc) need to be in containers that are 3oz or smaller and must all fit inside only one quart-sized plastic bag. You will need to remove these from your bag and put them in a bin (it can be the same bin as your shoes, but not with your laptop).
7) Remember to have keys and change out of your pockets before going through the metal detector.
8) Unless you keep large pieces of metal in your wallet (or have a magnetic closure), there is no reason to put your wallet in a bin. All that is in it is plastic cards and paper, none of that will set off the detector. I have never had a problem from a belt-buckle or from my wallet.
I fly over 100,000 miles each year for my job. I sat down last year and figured out that I spend more time in airports and on airplanes than I do at my own house. I know the process for domestic flights like I know the back of my own hand.
EDIT: I'm assuming cold-cut sandwiches. Obviously peanut butter and jelly would not be good because of the "liquids and gels" rules.
ZV