1. www.traveljungle.co.uk - gives you an indication of the cheapest flights on specific dates and which website to go to
2. www.airline-network.co.uk - once again gives a list of airlines and tells you whether the cheapest fare is available
3. www.travelocity.co.uk - click on the flights tab and select the option view cheapest fares and indicate which month you want to look at - it then allows you to check dates for that fare
Another thing is that once I've found the cheapest option I go to the airlines own website as it is often cheaper with them. Check the latest discount brochure for the airlines that offer serving and former srvice personnel reductions - BA, Virgin, bmi,etc
It all depends to some extent where you want to go in the States, and how cheap you want. If you don't mind and want as cheap as possible, I believe the you can even check with the crabs about indulgence on the London to DC "trooper" (Actually a Virgin Atlantic flight I believe).
It overall depends on where you want to go. But an easy way is travelling to Canada for example say Vancouver BC if you desired to head to Los Angles, from there jump on a flight down to LA which would cost you roughly Â£50. If you also do not mind the wait try flying into canada, but hoping on Grey Houng bus, extremly cheap.