I took Rick's advise seriously about making reservations for some of Italy's most populars sites. My husband and I are visiting Rome, Assisi, Florence and Siena in June 2008. We used tickitaly.com to reserve dates and times for our five must see sites: Borghese Gallery, Colosseum, Vatican, The Academia and the Uffizi. It was so easy! We received vouchers confirming the reservation, the reservations are paid for (one less bit of travel funds we'll have to jam into the money belt!), and no time will be wasted standing in long lines. It also forced us to build an itinerary while keeping enough time in between for moving about at a leisurely pace.
One tip if using tickitaly.com - have all your dates and times organized so you can navigate efficiently through the site. Also, there is a booking fee which some have argued stinks. But I was willing to pay the fee to be assured entrance. I haven't spent all this time and treasure planning this trip to miss our must sees!
Hope this helps.