On Sundays when I visit the Gurdwara, the bus drops me off just past The Grand Palace.
Location of Grand Palace
If you get into a taxi and the driver tells you that the meter is broken, get out of the taxi. The meter isn't broken, the taxi driver is trying to get you to pay more than the meter rate for your ride, and usually a lot more. Arguing is futile, and bargaining will only get you a price slightly less astronomical than the one the driver quotes you to take you wherever you want to go. The only exception — Phuket, where there are so few taxis everyone is at their mercy and rates are astronomical!
Some merchants will automatically charge foreign tourists more than they charge local Thais, whether they are selling fruit in the street, hotel rooms or t-shirts. Unfortunately there is rarely anything to be done about this scam, other than just walking away and finding another person to buy from.
If you know of any other scams, please do comment here and let others know.