I no longer fly, but when I did I'd hop on Expedia and search for flights. When I found one that looked good, I'd go to the airline's website and see if I could get a better deal there. Sometimes I could.
Cheapo tip:
If you fly out of BWI, there are a number of hotels that will let you leave your car there if you spend a night with them. So you can stay the night near the airport for less money than it would cost you for long term parking, AND you can take an earlier flight (cheaper) because you'll only be a couple miles from the airport and the hotel shuttle will transport you.
Most people who travel know that, but I thought I'd toss it out there anyway.