If a branch is available in your area and you are under 25 I recommend STA Travel.
www.statravel.com
I've used it for trips to Australia, Hong Kong, Toronto, Taipei and the prices have been better than any internet price I was able to find, usually by around 20 percent or so. Also, you can change the return flight date for a $35 fee, within a year timespan from the departure date.
I've never used it for domestic flights though, so I dont know how good it is for that.