Try not to look like a tourist. You're probably going to look like a foreigner, but that does not mean you need to look like a tourist. Do not bring a clutch of shorts and T-shirts -- Europeans don't dress that way, and you will be pegged as an American or Canadian immediately.
That said, you do want sturdy clothing you don't care too much about -- you will either destroy it with hard travel or be sick unto death of it by the end of the trip. Cotton shirts are fine, but synthetic blends are preferable for both tops and bottoms -- they dry quickly and don't need ironing.
Types of outfits you want:
Concerning shoes, bring two to three pairs, all of them comfortable. Unless you're spending all your time in the north, at least one pair should be sandals -- that goes for male travelers, too. Have walking shoes for big site-seeing or trekking days.
In addition to your outfits, you'll need your necessities such as socks, underwear and night clothes. If you are staying in hostels, bring modest night clothes. You also need a jacket and rain gear -- a water-proof jacket is preferable. Bring a scarf for additional warmth.
Remember: You're going to be hauling these items up and down stairs and hills. Pack your backpack as a trial run and go around the block. You will almost certainly downsize!