I definitely advice looking at Kayak.com. It goes through a host of airfare websites and provides the cheapest rates. Vayama.com is helpful too. I have found a lot of cheap rates through these websites.
Remember to clear out your cookies. A lot of times, airline websites will save the records of you visiting their sites and it affects the rates that you see.
I got a quick usefull information for peaople travelling to london. If they any London Airport Taxi Cab Transfer Service than they can try Hummingbird Cars http://www.airportransfers.net/
They also provide 5 % Discount on all Return Journeys also.
Off the top of my head, I would go to STA. It is considered the website for student travelers. I'm not sure if/how much more affordable it is than say a regular travel website, but it is worth a shot. I would also try hipmunk and kayak. They are both good sites as well. In terms of buying your ticket, it is common practice to buy it in the middle of the week (tue-wed). This is often times when airlines release sales. Hope this helps!
