Southeast Asia, like the Indian subcontinent, is affected by an annual pattern of monsoons. Monsoons are powerful winds caused by seasonal shifts in the atmosphere. Most people associated monsoons with rain, but this is only part of the story. In Southeast Asia the annual monsoon cycle begins with a hot season from mid-spring through mid-summer - temperatures is Bangkok can easily exceed 100 degrees during this period. The wet season then brings torrential rains from August to late October - this is the peak of the Southeast Monsoon. By early November, the dry winds of the Northwest Monsoon carries cooler, dry air from western China and the Himalayas. This relatively comfortable period lasts through winter into February and March, when the cycle begins again.
With this pattern in mind, we concluded that the dry season would be the best time to visit the area. Granted, pratically every other tourist takes the monsoons into consideration when traveling to Asia, so pleasant weather often translates into more crowds. Either way, we'd rather be dry than wet, so we selected November as our month of choice.
It's often difficult to get airline tickets for international flights very far in advance, especially since many ticket prices aren't even set until three or four months before your desired flight date. In order to find flights for November, I started to look for fares in July. The end of fall and mid-winter are both affordable times to fly abroad - you can usually find significant discounts for these periods. For example, a roundtrip July flight from Washington DC to Bangkok would have cost around $1000 from a discount travel agent. In February, that same ticket might be as low as $750. Cheaper fares to Southeast Asia are always cheaper from the West Coast of the US as well, so you may want to factor that in your search.
In the end, we decided to buy our tickets through a cyber auction sponsored by Hong Kong's Cathay Pacific airline. A cyber auction is an online bidding process where anyone can place bids for a product or service (in this case, airline tickets to Hong Kong). Several of our bids in this particular auction were accepted, the lowest of which was $600 round trip from New York to Hong Kong. Cathay also offered a $100 round trip excursion fare to Bangkok, which increased the ticket to $700. Finally, we had to find our way from DC to New York and back in order to take advantage of the special ticket offer. This added another $140 to the ticket, leaving us with a final round trip ticket price from DC to Bangkok for $840, which was an excellent price for November travel.