Buy packing cubes in an assortment of sizes, that way when you get to your hotel you can pull out what you need easily without unpacking your entire suitcase.
Big 3 - 3 pairs of pants, 3 sweaters, 3 shirts, 3 sleeveless shirts or camisoles so you can layer your clothing according to the weather conditions
1 pair of black pants in case you have to dress up and 2 other more casual pants
2 pairs of shoes (bring 1 pair of sandals if your trip is during the warm/hot weather) - walk in them for at least a few days before your trip to break them in.
Underwear-4 or 5 pairs (you can wash them out in the bathroom sink) Think about investing in a pair of travel underwear (REI brand is my favorite) for when you need your underwear to dry in 2 hours.
Socks-4 or 5 pairs (the soft padded kind are great), socks can take a while to dry in a hotel room.
Dresses, suits, skirts optional, I've stopped bringing a dress because I never wear them.
Travel money belt-absolute necessity so you don't have to worry about pick pockets. I prefer the ones that you wear around your waist or down your leg made by Eagle Creek.