It all starts with the suitcase. If you're dropping in to a country for a few days, you'll want to go carry-on only. This doesn't mean you should use the largest carry-on your airline allows, though! There are two reasons for this: first, you have to carry it. Bringing as little as practical in a small lightweight maneuverable bag will make your passes through security and trips on public transportation easier. Second, not all airplane overhead bins are created equal. For instance, United flies four different types of planes overseas. The maximum allowable sized carry-on fits just fine "wheels first" in a 777's overhead bin, but must be turned sideways in the shallower bins on a 767. I've seen this lead to chaos during boarding, as bins fill up faster while confused passengers struggle to stow their belongings--many which had fit just fine on their last trip on different equipment. A smaller bag will fit "wheels first" in the space leftover from someone else's sideways-stowed bag.
I am a huge fan of the smallest-size hard-sided four-wheeled "spinner" bag available at Brookstone. It's easy to push ahead of me through public transport turnstiles, and it makes a good footrest when sitting around a waiting room. I've even slid it under the seat in front of me on a train (or airplane after take off) to use as a footrest.
Now that you've got a bag to hold your stuff, you need to work out how little you can pack. The best piece of advice I ever heard on this subject came from a stranger on the parking lot shuttle at Newark Airport. She revealed that she never takes more than two pairs of shoes on a trip: the pair she has on her feet, and a second pair, if needed, in her suitcase. I've found this to be very helpful in putting together a tightly mix-and-match trip wardrobe. This approach can be extended to other clothing articles: wear one pair of pants, pack a second, wear one outer layer, such as a sweater, and pack a second (like a lightweight water-repellant jacket).
I'm not suggesting that you go gnarly and underpack to the point of discomfort. Everyone has their limits. I am not a fan of washing out my socks and underwear in the sink, so I always pack enough of each for the trip. I avoid anything that will wrinkle, generally sticking to colored t-shirts layered under knitwear. For a short trip I'll bring a clean shirt for each day. For a longer trip I will pack half the shirts required and air them out and re-wear them once.
If I'm traveling to a place where I know I'm going to do some serious clothes shopping, like London, I will pack less than I actually need. That way I can pick up an outfit or two while there, wear it, and have room in my tiny suitcase to get it back home.
I like to be as fresh as possible on the return plane trip, so I plan to keep one shirt unworn until that day. I've even packed that "last day" shirt (and a pair of undies and socks) in a ziplock bag to keep it away from my dirty laundry.
In general it really helps me to lay out all the clothes I'm packing on my bed before putting them in to my carry-on. That way I can see goes with what, and can eliminate any outliers. When it's all out there I then work through the days in my head, making sure I have enough of some items while not inadvertently packing anything extra.
One last tip on underpacking clothes: I've found that having ALL my laundry done before starting makes a huge difference in putting together a tightly-packable travel wardrobe. It saves me time in looking for items, and provides me with the greatest freedom of choice because nothing is ruled out because it's in the laundry bin.
Minimizing toiletries is essential for light packing. If you're feeling brave, you can do without everything and pick up what you need in a local store after arriving. You can either toss your new finds before heading home, or bring them back as souvenirs. I've gotten my liquid/gel toiletries down to travel-sized bottles of my shampoo and conditioner, a tube of mascara, a small tube of toothpaste, and a tiny tube of suntan lotion that doubles as my moisturizer for both hands and face. For everything else I use solids or powders that the TSA doesn't fret about--solid deodorant, and mineral-powder makeup. I sometimes don't even bother with the makeup.
Don't forget the charging devices for any electronics you'll be bringing, including your phone, camera, and/or laptop! If you can get a plug adaptor in advance bring that too. Organize these small things into ziplock bags to reduce the chances of them falling out when you open your carry-on.
Remember to pack your purse (or if you're a guy, your messenger bag) too. Dump everything out, see what you really need for the trip, and re-pack accordingly. Check your wallet for overload, too. You won't need to be hauling around crumpled up ATM statements, fourteen pens, or that book you keep forgetting to return.