I have not actually done this so I cant vouch for it, but I have heard that the airlines post their international fares 5am wednesdays, so if you want the lowest fare, you should try to buy it first thing Wednesday mornings, as they only sell a few seats at the lowest price. Its a bait and switch, but somehow legal. You will see American or someone else with super low fares, but when you try to get them, they are never available.
The travel agents that buy tickets and then resell them are called consolidators. Any good travel agent knows how to get in touch with these people and if you already have a good travel agent they can get you consolidator tickets. If you are flying out of an international hub, this works, as I have done this before: (It works for cruises, too) Buy a local paper, like NY Times LOCAL edition or Miami Herald, and look in the travel classifieds. those small ads with the low prices are consolidator fares. There are some disadvantages and restrictions to consolidator fares, but they generally are a good deal.
You can also fly with charter airlines. apple vacations puts together air/hotel packages and charters planes on a regular basis. You can buy the airfare only part and it tends to be pretty cheap (I hear their planes suck, though) For example, you can fly Cincinatti to Punta Cana non-stop for $399 round trip every saturday (last time I checked was 2 months ago)
