KEEP IT TIGHT
For a light packing job to work density is the key. Folded is smaller than wadded. Folded and rolled: smaller still. If your clothes take up more than one-third of your bag, you are probably taking too much.
THE RULE OF THREE
Three pairs of socks. Three pairs of underwear. Three shirts. Wear one, wash one, dry one. You can get more miles out of leg wear, so two pairs of pants and one culturally appropriate pair of shorts or a skirt should suffice. Choose light, flowing, quick-dry cotton-poly blends in matching colors that handle wrinkles well. For all other accessories like ties, watches, belts pick one that goes well with what you are bringing.
ROLL DON'T FOLD
To get the most space in your bag always roll. Tightly rolled clothes take up less space than folded ones. Plus, they're less prone to getting deep wrinkles from fold creases.
To get the most use from the least amount of clothes pick the right fabrics. Seek out material that resists wrinkling and stains, stays fresh-smelling after several wears and dries quickly. Lightweight merino wool for outerwear, quick-dry synthetics for underwear and SmartWool socks make life easier on the road.
Books are bulky, but bytes don't weigh a thing. If you're bringing a phone or other mobile device, consider downloading maps, translation apps, travel guides and reading material for your trip. One exception is travel documents. While it's a good idea to store digital copies of your passport and itinerary on your phone, you should pack a few printed versions as well. Still everything else should be on your phone or at the very least your tablet/kindle.
Thanks to www.lonelyplanet.com for the "rule of 3 tip"