I travel internationally quite often, and ASAP Tickets is one of the best services for great rates for flights.
At first glance I thought this was not a legitimate service however I've used them about seven times and each time I've been rather impressed. Some agents are better than others. My favorite is Fiby as she is very direct about when to purchase tickets and get the best prices. She has guided me the best.
You pay a small fee for a tip but it's worth the savings that you get for your flight.