Here's some lowdown:
Currently, Southwest airlines is offering great fares for ALL domestic flights. ALL one-way tickets are less than $124 (some are as low as $28), but these must be purchased online at southwestairlines.com.
I personally use travelocity.com when booking flights. They have spectacular deals, and allow one to choose flight departure/arrival times. Many other sites, such as priceline.com, etc., do not allow you to see the departure/arrival times until AFTER you have purchased a ticket. If you care about what time you are arriving at a destination (as you should with interviews), then DO NOT use one of these sights.
kpax, for the majority of situations, choose the cheapest fair. The cheap fair is a result of several things, the foremost of which are good business practices by the airlines, and stops in other cities enroute to your arrival destination. Many airlines simply have higher business costs, and hence higher ticket prices.
I flew last month on a combined Delta/Northwest airlines flight. On travelocity.com the Delta flight cost $200 more round-trip than did the Northwest flight, however it was the EXACT same flight (same plane!). Look around for the best deals. Right now, those are most prominently found on southwestairlines.com.
