Bangkok's International Airport is about 19 miles from central Bangkok. With the new airport rail link plus a slew of other transport options, it's quick and easy to get from the airport into the city even if you are traveling during rush hour. If you are, consider taking the rail link or a taxi or bus to a convenient Subway or Skytrain station for the rest of the journey.
Airport Rail Link
The Airport Rail Link is the cheapest and fastest way out of the airport and into downtown Bangkok. You can board the train in the basement of the airport. The express train will take you directly to Makkasan Station in about 15 minutes. From there you can take the City Line (the local line of the Airport Rail Link) two stops to Phayathai Station, where you can connect to the Skytrain.
Metered taxis are the easiest way to get out of the airport and to your hotel. Once you clear customs and immigration, head downstairs to the first level, there will be a taxi line outside. You'll stop at a desk and tell the clerk your destination and then he or she will assign you to the next available driver. They can help explain where you need to go if you but it is best to have your destination written out in Thai (ask your hotel or guest house to email you directions). You must pay an additional 50 baht above the meter fare, plus any tolls.
The fastest way to get into the city is on the expressway, tolls will cost you an additional 70 baht or more depending on where you are going. Most taxi drivers are honest but some will try to give you a flat rate to take you to your hotel. Insist on the meter!
Airport limos cost about 3 times the price of a taxi but are very comfortable and drivers almost always speak enough English to get you where you need to go with no confusion. If you have a lot of people or stuff, you can also get an airport limo minibus. If you opt to go that route, there are airport limo desks inside the baggage claim area and once you clear customs and immigration.
Express Airport Buses
There are express buses running from the airport on four different routes into central Bangkok including Silom Road, Khao San Road and the Sukhumvit Area. Buses run from 5 a.m. to midnight and you can get tickets (150 baht per person) at the airport bus counter on level 1 of the airport.
Public buses run 24 hours a day from the airport and cost 35 baht per trip. To catch one, to take the shuttle from the terminal to the transportation center. There are 12 different routes from Suvarnabhumi Airport to various parts of greater Bangkok. There's also a government bus that will take you straight from the airport to Pattaya (for about 130 baht).