If you are renting a car, picking it up and dropping it off at the airport, could save you taxi money. But here are some useful information that will help you cope with the chaos in major airports.
There is a designated drop off / pick up area at airport parking garages. You just need to spot the car rental signs and then find your company.
All major car rental companies have offices at big airports. The bigger the company your rent from, the bigger the space they have at their airport and the easiest it is to find them. So if you are renting from Hertz, they usually have the most prominent location at the car rental garage. Avis is the second.
Inside the airport building, there is also an office where you should fill all the necessary paper work upon arrival. Sometimes they offer upgrades for free, depending on availability and your luck.
If you are renting from the airport, take some time to set up the GPS before you leave the garage. Make sure your input is correct and the GPS will do the rest once you get out and it has a signal (GPS does not work in most parking garages). Check the language of the GPS to make sure you understand what it says. If you have any trouble, call the person working for your rental company for assistance before you leave, otherwise it is difficult to return.
Since you have to return a full tank, use the nearest gas station to the airport to do so. Usually there are gas stations very close to the airports, but do allow an extra 45 minutes to locate everything.