As a million miler, I've covered quite a bit of ground (or air) and, frankly, I've done most of it toting my own luggage and lifting it into the overhead bin on my own! That said, depending on the season, where I'm traveling from and to, what events I'll be attending, and the length of the trip, my actual items vary. However, the strategies for packing lightly but thoroughly always remain the same:
I review my travel itinerary so I know what I'm doing when, where and with whom; and can plan my wardrobe to avoid over- or under supply. Know the weather because it's not good to be caught out in the rain or to forget that January in New York is cold while Marrakech is a pleasant 70 degrees. Use paper or technology to chart your outfits ~ it'll make it super easy to see crossover, or under-utilization of certain pieces. I like a paper packing list like this (pack it up list) but also dig online resources like this or this. Avoid drycleaning and ironing by traveling with synthetic materials that can be handwashed and/or carry a steamer (this is similar to the one I own). Depending on where you're going and your purpose, leave a little room in the luggage, or bring an extra bag for goodies and travel booty ~ I personally never pack my new stuff in a bag that will be checked because I want to see my booty and play with it!
Travel stylishly and easy, and hopefully a little more stress-free.