Before you start your trip, ask your taxi driver for an estimate of how much it will cost. You don't want to be unpleasantly surprised when you get there. Be aware that what the driver tells you is just an estimate, so don't bust his chops if he's off by a few bucks.
If you're scheduling a taxi pick-up, make absolutely certain that you include enough time to reach your destination. It's a good idea to call the taxi company in advance, and ask them how long it will take. It's vital that they know the time of day, too, so they can account for traffic.