To get the best deal of purchasing any merchandise, you have to shop around and compare prices. Same goes for airline tickets. If you want cheap airline tickets then compare the price among several different airlines. This task could be easily done by using some of the famous online travel sites, such as Orbitz, Travelocity, Expedia, Cheap Ticket, CheapAir.com, Booking Buddy.
Some times, you have to search different travel sites to get the best result. However, there are sites that search 100 plus travel sites at once and find the cheapest fares. kayak.com is one of them. My favorite is itasoftware.com
To get your airline ticket at cheapest price, you would need to know the historical price of your itinerary. Once you find your airfare at or below that price then you know that is a good deal. FareCompare.com will do this for you. On FareCompare.com, you will find cheap plane tickets, discount first class airfare, the easiest air travel planning tools. FareCompare.com monitors airline ticket prices so you will be first in line to pickup the best deal on your quest for a cheap flight. On their result page, under Quick Links you could view historical deals, and subscribe to Alert Me on Price Drops feature. FareCompare.com will email you when your travel itinerary has dropped.
You can get cheap airline ticket even cheaper by using airlines coupon, others may call them voucher. Northwest Airlines distribute the zone voucher to the general public. I used to get them in the Val Pak coupons envelope. Continental Airlines gives out dollars off voucher based on the round trip airfare. Recently they are partnering with Chase bank to run the promotion. United Airlines compensate customers with $50, $75, and $100 voucher.
People are putting these vouchers on eBay, sell.com, craigslist in exchange for good or monetary. Some of them have no cash value (just like grocery coupon), you pay for their time to collect and send you the vouchers.