I’ve been on the road for about 8 months at this point. The whole time, I’ve never had more than one small 35-liter backpack, and I’ve never been inconvenienced by a lack of stuff.
Here are some tips you can use to lighten your load. Use them and then maybe someday you’ll be showing off how small your backpack is to everyone, like I do (a lot).
1. Only pack thin, synthetic jackets.
Why do backpackers insist on wearing thick wool and fleece jackets the size of a small child? Any kind of non-synthetic jacket is going to end up taking up way too much room in your bag, and they’re not necessary to keep you warm.
I spent two weeks in Ushuaia, Argentina, which is the closest “mainland” city to Antarctica. It’s cold there. Here’s what I wore, and I never had any issues, even walking around at night:

Posing in Ushuaia in front of my favorite pile of debris.
2. Bring shirts that are thin and less than 100% cotton.
I wear t-shirts from American Apparel (god they’re comfortable), and merino wool Icebreaker shirts. Both are thin and fold up very small in my bag.
My American Apparel shirts are 50% cotton/50% polyester, while the Icebreaker shirts are 100% merino wool.
I’ve bought 100% cotton shirts before and they ended up taking up twice as much space in my bag, if not more.
3. Fold and pack your clothes like a pro.
I wrote an article on what I dubbed the “Rational Packing Method” that shows you the best way to fold and pack your clothes.
Basically, put your heaviest, thickest items on the bottom — jeans, in my case — and then pack everything else tightly with rubber bands and ziplock bags, organizing them to all fit together like a puzzle in your bag.
Pack like you’re on a mission and you’ll end up fitting twice as much stuff into the same space.
4. Take advantage of the versatility of your laptop and/or cell phone.
I love my iPhone. It not only works as a phone, but it also serves as my Spanish-English dictionary (with the help of an application from the App Store) and my music player.
A lot of today’s smart phones can give you all the same benefits.
I lug around a 13″ MacBook but you could get just about all the same advantages with a cheap, tiny netbook. I read books, watch videos, and do just about everything on my laptop.
5. Re-bottle your toiletries, and toss the unnecessary.
I’m very white. I need sunscreen. The problem, though, is that sunscreen consistently comes in comically-sized tubes.

These containers are annoying.
What I recommend is buying smaller plastic containers, and just putting what you need in those. You can do this with sunscreen, toothpaste, shampoo, or really any toiletry you need.
Or even consider ditching some toiletries. I haven’t shampooed my hair for months now and am loving it.
6. Bring a maximum of two pairs of jeans.
I like my jeans. The problem, though, is that they’re thick, heavy, and take up a lot of room when being packed.
If you’re style-conscious, then pack a maximum of two pairs of jeans: one dark pair, and one light pair. No one will notice. Wear one of the pairs of jeans every time you’re traveling with your backpack.
7. Question every item you pack.
I have a thing for notebooks. I don’t know why, but I do. I hardly even use them. I always think I’m going to write down amazing thoughts, but I don’t.
I had three in my bag until I really stopped and questioned myself. “If I didn’t have two of these notebooks, how would it negatively impact my trip?” The answer: it wouldn’t. So I’ve tossed two of them.
Do this with everything in your pack, including extra clothes. You don’t need eight shirts — pack just the few you love.
8. Only bring one pair of shoes, no matter the conditions.
You don’t need both “casual shoes” and “hiking shoes.”
I went on a 5-day trek through the Colombian jungles in November, 2008 with my buddy Derek. Take a look at the shoes he wore:

Sporty, yet casual.
We hiked through rivers, miles of muddy trails, and uneven rocks. He did just fine.
And in fact, he still wears those same shoes to this day in the gym, out on the town at night, and really in any situation.
You only need one pair of shoes.
9. Use versatile shorts for everything.
If you bring a good pair of thin, water-resistant shorts, you can use them for working out, swimming, and casual day-to-day activities. That means you’ll only need one or two pairs of shorts total in your bag.
Anything else?
Leave any other light packing tips in the comments here.