With a bit of technique and pre-planning, it's pretty amazing how much stuff you can actually carry, safely and non-exhaustingly, in a backpack. On the other side of the coin, it's also true that you need almost as much stuff for a single night in the backcountry as you do for a weekend trip. The gear doesn't change much for one night or seven... just add some more food, first aid supplies, and you're off.
So, what's the trick to make it happen? Simple efficiency, of course. The heaviest items should go near the bottom, so your hipbones can carry most of the weight. Compression sacks help squeeze the air out of bulky items, and can add some weather proofing in the process.
Backpacker.com's Abbey Dufoe offers an overview on how to pack it all up, distributing the weight, maximizing comfort, and actually fitting all that stuff in there. Check it out:
How to Pack a Backpack Efficiently [Backpacker.com]