At the BKK Airport (Suvarnabhumi International), there are a few ways to get to Khao San Road. There is now a metro line that will take you out of the airport and into town. However, you then need to get off of the metro and take a taxi to Khao San Road. The cost of the metro and taxi together is more than just taking a taxi right away from the airport. We always take a taxi from the airport and insist on them using the meter. If you share the taxi and have 4 passengers, the cost of the ride to Khao San Road is about 280THB. Make sure that the taxi does not take the toll road.
There used to be a local bus that would take you near Khao San Road and then you would just have to walk, this service has been shut down (current as of March, 2012).
To get to the airport, the cheapest way (oddly enough) is with the tourist van. The cost per person is 120THB. Unless you have 4 people to share a taxi, this is a great price. The minivan goes every hour on the hour, has a/c and takes the toll road (making the ride only about 20 mins long). You can book this at any travel agency.
To get to any of the islands, you can arrange a boat & bus combination ticket at any travel agency. This seems to be the best way to do it. It’s very organized, efficient and a good price. We went from Bangkok to Koh Phangan by a/c bus and speedboat for a cost of 500THB per person.
You can also arrange for onwards transportation to Cambodia, Laos and Malaysia.