A popular service pushed by most shops is the fuel injection cleaning. It uses very strong chemicals to thoroughly clean out your injectors, the tips, valves, and even inside of the cylinder. Caution should always be used whenever around or using these chemicals. This has several benefits such as restoring power output of the engine, restoring fuel mileage, and reducing vehicle emissions back down to acceptable levels. The going rate in most reputable shops is around $90 to $120.
Generally, a reputable shop will have this service in at least a three step process all inside of a kit. The first step is a throttle body cleaning. It generally comes in a spray can like a bottle of carburetor cleaner. This stuff is extremely potent and it eats the carbon deposits off the throttle plate and idle air control valve. It is also the most labor intensive step because you have to take off the intake air system off to get to the throttle body. Sometimes, carbon buildup is so excessive that you may have to remove the throttle body to actually clean the back of the butterfly plate.
Trained personnel can use this chemical on throttle by wire systems but, for the average Joe, extreme caution should be taken. Be sure to check the service manual for your particular vehicle before servicing a throttle by wire throttle body. In most cases, the service manual calls for you to remove and unplug the assembly. It should be noted whether or not the PCM needs to "relearn" the electronic throttle assembly after pluging it back in. The chemicals can cause damage to the sensitive throttle plate motor and forcing the motor to turn while it is still plugged in could severely damage the throttle motor. After the throttle has been cleaned, the vehicle may be hard to start.
Step two is either the induction or rail service. Most kits only included one or the other.
The induction service feeds a chemical into the intake so that it will clean the intake manifold, the injector tips, valves, and combustion chamber. Depending on the tool used to administer the chemical, it can either be vaporized with air pressure in front of the throttle plate, or fed through a vacuum port directly into the intake. These methods are not very effective at cleaning the inside of the injector itself.
The rail service actually uses an adapter on the fuel line to inject the chemical directly into the fuel lines so that it will go into the fuel injector. The injector then sprays the chemical just like fuel. This effectively cleans the injector and all the little holes it sprays through, as well as the valves and combustion chamber. This method is not very effective at cleaning the intake manifold itself.
The third step is as simple as putting a additive in the tank. This is a very light chemical compared to the rest and is meant to be mixed with fuel. A small bottle can easily treat up to 15 gallons. Don't get too confused though, as this additive is a bit stronger than any that are in your local parts store.
Overall, the fuel injection cleaning service is a pretty good deal considering what all it does. However, this service usually doesn't fix many problems if they have already presented themselves. It can restore the vehicle to its original state, but will never actually improve fuel mileage. It doesn't take very long to get done.. give them say 30 to 45 minutes to do a good job. The induction service needs to be administered at a slow pace, otherwise converter damage may result. The high profit margin of the service also gives you an excellent chance to negotiate or haggle a better price with most chain shops.
Automotive parts stores also carry do it yourself service, check out Fuel Service in a Bottle.
Common brands used in the professional world is Wynns, QMI, Valvoline, and 3M products. A lot of third party repair facilities use them, as well as some genuine dealerships. Here is 3M's Fuel System Cleaning Kit