Don't say ok to the driver after he told me your first price quote.
Here's an example of what I heard from these European travelers the other day. I couldn't hear where they want to go
Tuk Tuk Driver : 400 Baht
Travelers : No, it's too expensive.
Tuk Tuk Driver : Ok 150.
Travelers : No, I am not going.
Tuk Tuk Driver : 100 Baht.
Travelers : OK and got on the vehicle.
Be careful of these Tuk Tuk drivers. They tend to take advantage of people who doesn't know the location they are going and make easy money. Not just foreigners, but also Thai people as well. A lot of good Tuk Tuk drivers are out there, so I am not blaming the good ones just the bad ones that made Bangkok a horrible feeling.
I am Thai and I am really ashamed of what these people trying to do.