One mistake that most people make when packing for their trip is bringing too much. You start packing thinking that you will probably wear all those clothes, when in fact you won't. When you first pack take 1/3rd of the clothes and put them back in the drawer. When you are traveling there is a low possibility that you will be going out to the hippest clubs every night. It won't happen, you will be way too tired. And another thing, no one will know if you wore the same shirt three days ago, so don't pack too much. There are laundromats in every city in Europe. I made a list that you can use as a guide.

Clothes to bring:
Two pairs of jeans
Some t-shirts (plain colored t-shirts are the best)
A waterproof jacket
One sweater
One nice button up shirt for guys or dress for girls
One pair of shorts
One pair of athletic shorts
Enough underwear
Plenty of socks (non-cotton socks are the best because your feet won't sweat as much
Comfortable walking shoes
One pair of light weight non-athletic shoes (most clubs won't let you in if you are wearing athletic shoes)
Bathing suit
Shower flip-flops
Remember too that you can buy anything you need in Europe.