In the past, that required having copies of a number of different reservations and complex trips could be overwhelming. A few years back I started using the Tripit Web site to manage all of the plans for each of my trips.
When you book a flight, you simply forward the reservation e-mail to Tripit and it will automatically be added to your trip. Ditto for hotels and rental cars. This makes it very easy to get all the major components of your trip added to an itinerary with a minimal amount of effort.
Other parts of the trip will need to be added manually, but it is still pretty easy. When all the information is entered, you only have to track a single itinerary. Tripit makes it easy as you can view it on the Web site, print it out, or use the app on either a smartphone or tablet. I always print a copy so that I have a hard copy with me, though I do often use both the Web site and the smartphone app.
Some of you may be worried about privacy. If you want the itinerary to be public, that is an option. But you can also make it so private that only you can see it. If there are individuals you want to be able to see it, you can invite just those people. Of course I leave a copy of my plans in the office in case I need to be reached.
So how much does it cost? The basic version is free and it does everything I need. There is a Pro version with a cost if you feel you need more features than what the free version offers. Get signed up and make your travel planning much easier!