If you travel often, it's nice to have a fairly automated way to keep up with trips. Tripit.com will collect and organize flights and other reservations (car, hotel, etc.) for you. The account is free and the iPhone app is also free. I've been using it lately and found it very useful.
You can forward the confirmation e-mail message from the airline, etc. to email@example.com and it'll automatically create and organize the trip information for you. Of course you can update things online but the forwarding e-mail feature saves a lot of time. [more]
You can access the trip information from your iPhone app - which is really handy when you're on the road.
There is also an option for Tripit to send you reminders when it's time to get a boarding pass, etc. Also, you can have it send you reminders (by e-mail, SMS, etc.) re: airline boarding gates, flight status, etc.