Istanbul taxis are bright yellow and have a "Taksi" sign on top. Don't use any other kind of taxi when you are visiting Istanbul.
All Istanbul taxis have meters. Ask the driver to turn the meter on before you get in the taxi and definitely before they start driving. Otherwise they might not use it and will charge you whatever they want to when you arrive at your destination.
Don't try to agree on a price up front instead of using a meter. No "deal" they will offer you will be better than the price from the meter, and often Istanbul taxi drivers will lie to you about the distance from one place to another in order to get more money from you.
Make sure to look at the meter in your taxi when it starts running. During the day and evening it should say "Gunduz", which means daytime. At night it should say "Gece". The night meters are only supposed to run between 12am and 6am and the night tariff has a 50% premium.
Many Istanbul taxi drivers will try to set the meter to "Gece" even during the day, so be very careful that you check the meter when you get into the taxi. Watch the meter for the entire duration of the taxi ride, as some drivers will try to switch it from "Gunduz" to "Gece" when you're not paying attention.
If you cross the bridges while in an Instanbul taxi, you will have to pay the toll. This is true even if you cross a bridge in the no toll direction.
The last Istanbul taxi tip is to wear your seatbelt at all times while in the taxi. Drivers are often aggressive and minor accidents are common.
