You could use EFax as an electronic in-box and your hard-drive as your filing cabinet--or your e-mail server could store paperwork. Anywhere you go you'll find a fax machine, and any paperwork could be faxed to your EFax number, and it could be viewed, sorted, printed out if necessary, or left stored on disk in e-mail. If you get an 800 fax number, it shouldn't be a problem to use the clients' fax machines without incurring long distance charges, or since most of your travel is local, just get a local number.
The clients' fax machines would be your scanner, and you would fax to the 800 number, which would go to your e-mail account.
I have a client who does this. The drawback is that they're converted into a proprietary format and are received as attachments, and you have to have the software on the computer you're using to view the files, but that's a fairly trivial issue.
It's really handy when he's out of town to fax paperwork to him, as he can get it wherever he is, even if overseas, and he can forward the documents, sort them, leave them on the server or download them to his laptop.
I think you may be able to accomplish this with Vonage, too, but I'm not sure about the details.
You have an audit bag you have with you all the time, don't you? You could use the the plastic folders and plastic envelopes that are available nowadays --they're really handy for keeping paperwork you pickup during the day--they seem lighter than the paper folders, and more durable and tidy. Some of the file folders have flaps that keep papers contained pretty well, and if you use color--one color per client and one color for your personal things, you will be able to keep papers sorted pretty well.
And of course, to reduce the paper you carry around to a minimum, you could just fax the papers to yourself and shred them afterwards.