Additional information regarding the Airport transportation:
There are two lines running between the Airport and Phaya Thai / Makkasan station. One is the Airport Express, the other is the City Line (citiliner). Both are running on the same track but one stops at each station and the Express will go all the way to either Phaya Thai or Makkasan station. The fare for Airport Express is 150b (return valid within 2 weeks) and 90b (single); the fare for Cityline is 45b between Airport and Phaya Thai. The journey time is 15 mins and 30 mins respectively. You will find the route map here.
Taxis: queue at hotel doors. Get the doorman to tell the driver where you want to go and be insistent that you don’t want to go anywhere else and you DO want to use the meter. It's often best just to walk out to the roadside and hail a passing cab yourself ALWAYS ask if he will use the meter if not, refuse the ride and try another taxi ,sooner or later you will find one who will use the meter. Inner city fares are rarely over THB100. Be prepared for the driver to have no change by always keeping a small stash of THB20's, 50's and 100's. It's a good tip to carry your hotel’s business card (which will be written in both Thai and English, for the return trip.Moreinfo - LINK
Tuk Tuk: you need to haggle hard and agree on the price before you get in, then only pay once you've reached the destination you asked for safely. They can be fairly unpleasant because of their apparent recklessness and you sit in the exhaust fumes of Bangkok. They are one of those Bangkok "must-do-once" activities though.
River Transport: Don't be afraid to use the river and khlong boats. How to and further info - LINK