Lightweight packs! This happens to be a favorite subject of mine.
If you're trying to lighten up your pack, try looking at every item you carry. Seriously, spread all that shit out and look at it. After a critical look, you'll notice most of the stuff we lug around is pretty pointless. Fun maybe, but necessary? Hardly. What do we need to travel? The basics. Here are a few major offenders that can be fixed.
Sleeping bag: Unless you're crossing the Rocky Mountains or traveling in winter, a 20 degree bag is more than enough. That shitty cotton bag that weighs 6 pounds? Fuck that, make a 2 pound quilt that will keep you warm even if it gets wet.
Shelter: Carry a small, 16 ounce tarp made of silnylon for sleepin' out of doors. It keeps the rain off and weighs less than any tent. For a ground cloth go rip off some Tyvek house wrap from a construction site.
Extra clothes: Obviously bring enough to keep warm, but consider leaving alot of redundant items. How many shirts do you really need? Pants? Not more than one, unless you carry a clean pair to wear while shoplifting (excellent idea, by the way)
Blah blah blah. The basic point is ditch the useless shit and carry what you need. If you get to an area there you NEED something you don't have, go boost it. Wasn't there a "gear list" thread on here? I'd be interested to see what you're carrying to determine where the weight is coming from.
Maybe my thoughts on this matter are skewed a bit, since I don't travel full time. I go out for 4 to 6 months and then return to a city to recoup. Sure, some folks out there are on the road 365, but even then I see folks carrying around the weirdest stuff and it weighs them down. Like the homeguards pushing around a shopping cart full of junk; just what the fuck do you need all that shit for? Lighten your load, and moving/running/hiding become that much easier.
I like Doobie_D's answer. Burying your shit in someting waterproof! Brilliant. And what Yolo says is so true; if you carry a big pack, you WILL fill it up. Carry a smaller pack and you learn to pack smarter. When I go out in the wilderness for months at a time, by pack weighs about 10lbs (without food and water) which is enough for anything I'll encounter from desert heat to snow and cold above treeline. My baseweight is a little extreme and I don't suggest you get THAT light without experience first, but I mention it to show it can be done. Remember too, I don't travel the whole year, so I store my shit in a tiny storage unit when I'm out and about.
I hope you are able to lighten your baseweight, your feet and back will thank you!