I don't think many regular buses go to KSR. If you want to go somewhere else on the BTS system (or MTS) the tourist buses are still probably a good bet.
If you want to catch a bus in the street they most buses will go to the Moh Chit BTS station -- this will get you the Skytrain and the underground station. Some buses take a long route or not go there, so ask "Moh Chit" or "Chatchuchek". I usually use the yellow bus #29 which runs till late at night. Many people will get off at Moh Chit. Usually, the conductress will tell you. From there you can get onto the Skytrain network. There is a lot of signposting in English.
Buses stopping on the same side as the airport go towards BKK. The underground train station also is located near the Moh Chit stop.
Paul


