All joking about my excessive Powerbook love aside, any laptop is a lot to carry if you don't have to. Since I work in three different offices in the course of the week and the budget still seems to think I should only get one computer, I pretty much have to.

I switched from a messenger back to the smallest computer backpack I could find after messing up my back last year; for the same reason, I was really pretty intense about minimizing what I carried for a while, and the discipline has stuck. Thanks to the multiple locations, my backpack is my office, but it's also as lean as I can make it.

So --
Backpack main compartment:
12" Powerbook + power adapter.
Large pad (pref. graph paper) for @work.
Small notebook (5"x7"-ish) for !@work capture.
2 folders, different colors. One is for loose papers I'm currently working with, the other for stuff that's in transit between locations.

Backpack side compartments:
Wallet
ADD meds
Video iPod, headphones, USB cable (I'd love to not have to carry the cable)
Cell
Pager
Work ID (when it's not clipped somewhere on my person)
Pens (1 ea. fountain pen, rollerball, mechanical pencil)
Keys (as few as possible)
Post-its

Pocketses:
Pocketmod
A coupla bucks

I don't carry:
Reference material. It's online, believe me.
Books. Much too heavy when balanced against the likelihood that I'll ever even get them out of the bag before I get home.
Organizer/Planner. Anything calendar-ish is on the computer, and I've never liked how they organize anything other than calendars.
PDA. I've tried, but I just can't get into using them.
Unnecessary papers. Fat printouts that I really only need to skim, projects I'm not actively working on, three-month-old pay stubs, other assorted paper crap that accumulates in the bag. This can be the hardest part.
Moleskines, hardbound composition books, or other fetishistic notebooks. I have big-time stationery lust, but for the backpack, I want something legal-pad cheap that I can rip out and throw away.

I capture on paper (at least for the stuff that doesn't come in by email), and a lot of draft writing and general project work happens on paper as well. However, I don't keep old notebooks -- captured stuff gets transferred into the computer if it needs to be saved, or crossed off the list if it doesn't, and then tossed. I go through a lot of pads of graph paper...