The last time for me was in April 2002, and went for less than US$350 with taxes on BA
, on a 777, EWR
. The favorable money exchange rates for you in the UK and EU and causing a booming demand for flights and raising fares.
Consider purchasing from 'bucket shops', there are many in London and advertise in the newspapers for foreign travelers, like in Earls Court with the Aussies/NZ's.
Consider last minute travel agent deals (Thomas Cook, etc) offering charter airline seats or charter seats on major scheduled airlines.
The further ahead you buy, the better the price you can get.
has offered cheap fare deals in May to fill out their seats in early summer, depending on demand.
Consider alternative airlines such as Icelanic or Air-India. Check out Virgin too.
Be flexible as to days/weeks/month can travel. You may be able to get a good price if travel on midweek days. April, May, September and October are shoulder months with more moderate fares.
Avoid US/UK holiday times. In USA that is the last Monday in May (Memorial Day, our main day for honoring war dead and July 4th, US Independence Day, when in 1776 we told you to go piss off! Also avoid early September, as the 1st Monday is 'Labor Day' an day off for almost all people. Early to mid September could offer good buys, unfortunately due the September 11th.
Consider going to/from different city pairs, such as Birmingham or to Philadelphia in US. By the way, AVOID US AIRWAYS, as in deep financial trouble.