I got a nice messenger bag from Banana Republic a few years ago. I had one similar to this one before but it ripped because the material was thinner. This one is more heavy duty and has lasted me without any problems. Its nice because it has a ton of pockets and zippers nearly anywhere and everywhere. I can put all pens, pencils, calculator, flash drive, post-its, etc, inside that pocket. Once its closed you can shake the hell out of it and nothing will fall out of its pocket.
http://img261.imageshack.us/img261/6823/pict0324ge1.jpg
http://img89.imageshack.us/img89/4786/pict0325el1.jpg
If you are looking for a new bag, look for overall ergonomics. Make sure it's got pockets anywhere and everywhere so you can put things in there and they stay in place. Make sure you shake it really well to see if things stay put. The older one I had before this wasn't designed as well. Even though it had pockets, things would fall out and not stay in place.
You can't see it in the picture, but its also got a zippered cargo net inside, a zippered pocket inside (shown in the picture), a slot with a strap that velcros to hold stuff in the pocket down (I put notepads in there). In the back is another open slot to put things, and a zipper runs along the length of the bottom to increase the width if you're putting something wider inside. It's really, really functional.