Although this might not interest the ladies, I have, through several trips to Spain, come up with a lightweight set of clothing which I can wash in my room sink, and, in summer, dry outside in a few hours (not difficult in the summer Spanish sun!). I tend to wear black, it dries quickest, can be dressy or not, and does not show too much dirt, certainly no stains. I pack two or three pairs of black Dockers cotton chinos, the same amount of black, synthetic loose, short-sleeved button shirts, black "ropa interior" and wear Clarks black comfortable shoes. By hanging them to dry, and carefully folding (I used to be in the Navy and Marine Corps) I don't have to iron. You could vary the color, I am a painter and wear black so it doesn't reflect on the canvas when I paint. Throw in a poly-cotton blend blazer and tie and you are set.
Be sure to wear super comfortable shoes, they have stone sidewalks here, and they really are harder than cement. Get some podiatrist insoles to support your arches too, you will be glad you did, especially if you have flat feet. I like black leather sneakers (look dressier and don't scream "¡TOURISTA!").
Another way of minimizing what you take, is to purchase over here. Take a change of clothing and buy what you need. You can get a refund on the VAT tax when you leave too by filling out a form. In Madrid, the Cortes Ingles department stores have all the US brands, and there are many, many clothing shops downtown, and just about everywhere. The shoes here are especially good. Spain is famous for its leather goods. Check out the cool coin purses, super functional.
Spanards wear basically what a well dressed norteamericano would, perhaps better shoes, and less screaming branded goods (except the kids, they are the same everywhere). Be sure to pack or purchase some good dress shoes, you won't be allowed in a club with sneakers, leather or not. Ditto for sports shirts and sports clothes.
You will find just about everything here, same brands. Toothpaste to shampoo, cosmetics etc. The cancer of our culture has invaded everyplace. Too bad.
¡Disfruta!
Martín de Madrid
President, 1st Class Pinups
"The Classical Alternative in Pinup Art"
http://www.1stclasspinups.com