This might entail walking the back roads of a city, away from the touristy-type areas, or you may want to steer clear of the larger cities all together and experience smaller towns and villages. You will also find that getting up early and visiting major attractions before the rest of the crowds show up will help reduce your queue time. Tour groups seem to appear out of nowhere on their large busses, causing major congestion as they swarm the hot spots. However, they don't usually like to get up too early and they take more time to get organized, leaving the morning relatively quiet for others. The end of the day is also usually a good time to see the sights, after the tour groups have moved on.
Accommodations are worth more than gold in the busy season.
Your best bet is to book beds in advance. Purchase a Hostelling International card before you leave so that you can make hostel reservations from city to city at any HI hostel. If you are staying at a private hostel, try to call ahead and to book a bed. By simply arriving in a city and expecting to find a place to sleep, you are setting yourself up for hassles and headaches. One travel strategy in Europe is to visit the southern countries like Italy and Greece in the early spring and then travel further north as summer approaches, using the improving weather to your advantage.