While Venice can be very crowded, especially in summer and during Carnevale, it's easy to get away from the crowds by getting away from the main tourist pathway that runs from the train station, past the Rialto Bridge, and on to St. Mark's Square. Wander the picturesque back canals and visit some of the lesser-known churches. Visit St. Mark's Square before the Basilica and Doge's Palace open and you'll find far fewer tourists. You'll also be first in line when they do open, usually before the tour groups arrive. According to Venice Travel Weather, the best times to visit Venice are spring and early fall.
Picture of Piazza San Marco © 2006 by Martha Bakerjian, licensed to about.com