I'm that girl that packs clothes that all coordinate to mix and match, and I only pack one extra outfit per trip. I even manage with only one pair of shoes most of the time! This article in the NY Times from a flight attendant, called "10 Days in a Carry-on" changed my life.
One recent investment I made was in some extremely lightweight packing cubes from eBags. I purchased their value set.
I love these bags. They weight virtually nothing and collapse completely. I used to pack outfits in gallon Ziploc bags! These little "cubes" are fantastic. You can roll clothes to keep in the larger bags, use the smaller ones for underwear/socks, and even keep one just for phone and camera chargers. It keeps everything from being completely jumbled around not only as you travel, but as you live out of your suitcase at your destination.
I even use this in my duffel bag when I drive up to visit my parents. It makes separating clean from dirty very easy too.
Another tip? I always make a checklist. It helps me sleep the night before a trip. I also keep duplicate toiletries specifically for travel: toothbrush, travel toothpaste in baggie, deodorant, etc. I've forgotten these items too many times before trips to count, and buying them at my destination when I know I have them back at home bothers me. So now I keep extra on hand.
Do you have any packing tips?