1 use the sky train, local bus etc if you can
2 if you use a taxi, agree a price before! They even put your suitcase in the boot, agree if the price includes toll fees or not ( if you ask it never will, so say including toll fees yes?)
3 find out what the average price for the journey is eg bkk airport to nana 350b,
4 When you get to your destination, grab your bags, give then the agreed money (NEVER give them before), if they start to argue, walk away. Make to too much hassel for them to chase you.
5 when you get ripped off, and you will, put it down to experience, taxies cost a third of what they do at home, think about what you didnt follow above and learn from it