The NYT had a piece on finding cheap airfare. Since I'll probably be making a trip to the States this June/July, the salient part follows:

ITASoftware.com, which provides the technological backbone for many airfare shopping sites, allows users to scan an entire month's fares for the least expensive rate. (Log in as a "guest" and click on "month-long search." ) In January, the 28th and 30th were the cheapest dates to fly nonstop to London from New York ($536) for a week's vacation, according to a recent search. The next best was Saturday, Jan. 23, at $640. To book the ticket, users must go to another site. Kayak.com has a flexible-dates option (registration is required) and a calendar that shows the best fares found by other Kayak users in the last 48 hours. Bing Travel, the Microsoft search engine, offers a similar option, found under "plan trips," about halfway down the page.