Driver dispatching is an art. In order to be an effective dispatcher, you must be able to visualize your area, your restaurants' and customers' locations, as well as each driver's current position.
The trick to great dispatching is being able to your drivers so that they arrive at the restaurant just as the food is ready for pickup. This actually becomes easier as your service gets larger. The more drivers you have, the better the chance that a driver will be available when you need him where you need him. (or her, of course!)
Most services use two-way radios (such as Nextel's direct-connect feature) to communicate with their drivers. However, recently many RDS's have turned to requiring the drivers to use their own cell phones. Text messages can be sent to the cell phones informing the drivers of pending orders.