ah, a travel question! i used to work in travel, i LOVE planning trips!
i normally go through ebookers. it also helps to know when the peak seasons are, namely july, august and beginning of september, and avoid them as much as possible.
luckily for me, we always travel to hawaii in winter (nov-beg. march, except the holiday season) so we get tickets at a reasonable rate. i usually fly united (goes direct from heathrow to honolulu) and they sometimes have special deals that make it the same as a travel agent.
other than that, i usually check
www.fodors.com/forums every so often, someone usually posts an airfare sale notice there. got tickets from los angeles to las vegas (roundtrip, southwest) for $30 each way. pretty good when it's normally like $50 o/w.
you can also try
www.hotwire.com, although i personally find better deals elsewhere. i found ebookers the best from travel from london to the US, then once you hit the US it's cheaper to find a US site. also, i have aol and they have 'keyword: bargainbox' and that'll list travel company sales, as well, though mainly US domestic or US-originating.
lastly, if you don't mind regional/budget airlines go directly to their website (like ryanair, southwest, jet blue, etc) as when they run specials you can only get 'em from the company direct. SOME (not all) travel agents don't bother with those fares because no commission (although i don't know how much things have changed since 9/11).
can you tell i book a lot of flights for my family and some friends?