You don't mention where you are flying from. If you are on the West Coast you have more airlines to choose from. Several Asian carriers fly from SFO and LAX. From Boston to BKK it is about 23 hours of flying going through Japan/Korea or Taiwan. (Not including the time waiting in airports for your flights!) If you are on the East Coast and fly through Europe it is a little shorter flying time but costs more money! You did ask for cheap flights – cheaper flights take longer than more direct flights like Thai Air!

You can help yourself to get a cheaper ticket by being flexible on your departure date. Check the price a few days before and a few days after your desired date for any price changes. Flights leaving on Fridays, weekends and holidays may be more expensive too! June through early September are the most expensive for flights to Asia but in Thailand the hotels are on low season pricing.

Always check the airline's websites for any special prices. You should check the airline's websites to establish a price that the wholesalers and bucket shops have to beat – otherwise you wouldn't know if you got a good deal or not! There are some good prices when there are cancellations because tour groups cancel out and the airline wants to fill those seats quickly! Check the websites often!

Remember to choose your seats ahead of time so you don't get the crappy seats. Get up often during the flight and bring a couple books to read. Complaining about the long flight doesn't make the flying any easier. If you have an elite card (silver or gold, etc. frequent flier card) use that airline so you can get bonus miles and more perks like priority boarding and better seats and free upgrades!

Plan on spending a few days in Bangkok at the beginning of your trip and end up there the last night of your trip so you don't have any conflicts with connecting flights. In Bangkok get a hotel near the Skytrain route so you can use that transportation service.

You can take the overnight sleeper train from Bangkok to Chiang Mai if you want. This will save you from paying for another night in a hotel in Bangkok. Nearest beach to Bangkok is Pattaya/Jomtien. The bus to Pattaya leaves from the Ekamai bus station (off the Skytrain stop) and will cost around 118 baht and take 2 hours. Only buy your ticket from the bus station.

Read up on Thailand on some of the travel links below for more detailed info.

Good luck.

There are several ways to get the best airfare:
Try one of the big companies: Travelocity.com., Priceline or Expedia.com. The advantage to them is that they show many different airlines and the cheapest prices for both the specific date you want to fly and the dates surrounding that date. In case your plans are flexible. The disadvantage is that you are locked into the fare dates. If you want to change a date due to injury or an emergency, their fees can be extreme.
My personal method it to use Kayak.com as my initial check on fares and dates. I generally start at least a month in advance when looking for a ticket. The farther in advance, up to 3 months, the better. The nice thing about kayak is that it hooks you up directly to the airline. Hence… you don’t get locked too deeply into one of the big three that charge up the wazoo for rescheduling. Then I figure one or two flights that are at the right times and right prices. In the upper left corner of the Kayak screen there is a feature that says “ track fares by email”. It’s referred to as Fare tracker and it really helps. Sign into this feature and it will update you daily on the changes in airfare. At least once in the month you are going to be able to find a fare that is usually a couple of hundred dollars less than what you have been quoted in the first place. A very cool feature.
Also, check out currentcodes.com and naughtycodes.com. These two sites have discount codes for some of the airlines and can save you some real money if you catch a special at the right time. They have discount codes for just about everything including airfares and Cruises. Not to mention coupon type codes for many stores and services.
If that is not what you want or you are in more of a hurry, then…
Try the airlines directly. Either their Web sites or direct telephone lines. Some of the Airlines offer low price guarantees meaning that if you find a cheaper fight on another Web site they may match the price or beat it. Each airline is different though, so you have to check them out individually.
Just remember, if you are going to try to get the cheapest flights, the airlines normally have restrictions. You may have to purchase the ticket 14 or 21 days in advance, there may be no refunds or changes allowed. You really need to know these restrictions.
If you try looking for a last-minute flight you will end up paying a lot more, although there are occasional exceptions. I personally, managed a last minute ticket to S.F on the Millennium New Years eve. And received a better price than my friends that had booked a month in advance! But… you cannot depend on this at all.
Usually travel agents do not get the lowest fares and sometimes they tack on a commission. Travel agencies used to be the best way to get the best airfare, but with the Internet, It is now just as easy for individuals to check themselves.
If you call an airline directly (they all have toll-free numbers), be sure to ask them if they have any specials. Often they do have specials, but you have to ask them about them, they won’t just tell you!
So be careful about restrictions on tickets. Be aware of charges for overweight baggage and remember that many of the very CHEAP fares you see in the Travel section of the local Papers are fares that are quoted, “without taxes” Taxes can really add up.
Have fun and happy travels.