Being the resident Polish dude, I'd say go easy on long-sleeved clothing, unless you need some business shirts for work (not likely if you're an Au Pair).
Apart from my office clothing, I dragged one single thin sweater with me. Turned out to be a waste of place in my backpack, I haven't touched it once in the month I'm already here.
The overweight is VERY expensive (34 euro per kg or something similarly insane), so if you want to do some serious shopping, better pack less, you can buy rather inexpensive clothes here, same goes for necessities like soap, shampoo and the likes.
A small umbrella would be nice, but you can buy them everywhere here.
Contrary to the common belief, electronics aren't much more cheaper here than in Europe. So if you already have a digi-cam, by all means brng it with you.
Maybe some more things will come to my mind later
regards
Mike