This tip works if you're taking a taxi from BKK airport to downtown Bangkok:
When riding tax into the city from BKK, you will go through 2 toll booths on the highway. Generally, the taxi driver will ask you to pay the Toll fee (it's normal, don't worry). Each toll is like 30-50 baht, but the best part is, they will accept your large 1000 baht bills you just got at the airport and give you change for it. Do this at each toll booth!
When you get to your destination, you'll have quite a few 100, 50 and 20 baht bills - much better for buying from the small street/market vendors, food carts, tuk tuk drivers, etc.