I buy all my trips on Priceline.com. For me they have always been cheaper than all other sites, especially if you bundle a car or hotel with your flight. If possible, regardless of the date of travel, look to buy your ticket on a Tuesday or Wednesday morning when prices are lowest.
I've used cheapticket.com and orbitz.com in the past.. I've never bundled as it seemed more expensive that way. Always buy on a Tuesday or Wednesday morning and it's best if you can be flexible with dates of travel.
I usually search online at all the sites, priceline kayak cheapoair, etc. Then I print them out and bring them to mytravel agent, who has so far found a way to get me something cheaper (unless it was a last minute trip, in which case I just buy them online). When tickets to England were averaging 700+, the agent got me one for under 600.