[Nyah]

If you're flying in & out of London and don't care about not getting free drinks/stuff on the trans-Atlantic flight, I always find AirTransat to be by far the cheapest airline. Air Canada, KLM and Lufthansa are great airlines for trans-Atlantic flights, with free booze, awesome meals and pillows/blankets on their flights, but they're also generally a LOT more expensive (in my experience). That being said, sometime you can find amazing deals with those airlines as well. I know Itravel2000 has flight-watchers where you can get email notifications of drops in prices on the flights you're looking at.

Edit: also, price out the flight you want departing on a Thursday or Friday. Those days I've found to be the cheapest to fly. I try to avoid doing big flights like that on Saturdays.