It doesn't matter whether I'm going on a trip for a month or for four days. My gear is the same. And my inclination is always to simplify what I carry.
The suitcase holds clothing and any cords or cables. My trusty Coach briefcase has front pockets for my wallet and my cosmetics bag. I carry a plastic file folder with printouts of boarding pass, hotel confirmation, any business-related materials, and a photocopy of my passport, drivers license, and credit cards. Because I post up to "Annie's Take" every day and the program to do that is Windows-based, I carry both a lightweight netbook as well as my iPad and iPhone.
I never check my bag. It saves time on both ends of the flight, but it also reminds me to keep it as light as possible for the lifts into the overhead bin. My briefcase goes under the seat, for easy access. I do not carry a purse, though I have a small flat one packed in my bag for occasions where I don't want to carry the briefcase, like dinners or sightseeing.
Given recent fiascoes with plane landings where passengers have had to use chutes to exit the plane, I've just double checked both type the types of planes (Boeing on the outbound, Airbus on the return) as well as my seat assignments (always aisle for me, and near an emergency exit as well).
On each trip I take, I plan in personal time. Since I'll be in DC for
four days, that includes meals with friends and colleagues as well as my
usual walks to the Mall to visit the Vietnam and Lincoln Memorials, as
well as to the new Martin Luther King Memorial. I'm going to write more about that side of the trip on my personal blog.