Yesterday I ordered a Roma Pass. € 80,00 for the 4 of us. We'll pick up the passes at the Colosseum. The 3 day pass provides free entry to 2 sites, 1/2 price entry to others and full access to the public transport system. It should be a good deal - plan to use it early in our visit.
It is still a bit amazing to me that I am able to do that from my computer at home here in Canada, with complete confidence that the passes will be waiting for me when I show up at the Colosseum in about three weeks.
Several weeks ago I booked and printed out our train tickets from Paris CDG airport to Avignon on the SNCF rail system in France. I just followed these very helpful directions. Our TGV tickets are tucked away with our passports.