By using some of the various aids below, you can properly tune the aerodynamic characteristics of your car not only for downforce, but also for cooling and stability. Some of these parts might be overkill in anything but competition, but all are readily available as bolt-on or home made parts for most imports.
We'll call automotive wings 'inverted wings' to avoid confusion with aircraft wings. Although aircraft wing sections are similar, the automotive version is inverted with the more curved surface on the bottom, hence the name. An inverted wing is a device that generates downforce by creating a pressure difference between the top and bottom wing surfaces. The oncoming air splits at the wing's leading edge, where some air goes over and the rest goes under the wing. Because of the wing's profile, the air going over the top is moving slower than the air on the bottom. In addition, Bernoulli's law states that slower-moving air possesses a higher static pressure. As a result, the higher-pressure air on top pushes down more than the lower pressure air on the bottom pushes up. This pressure difference, in addition to the plane-view surface area (angle of attack) of the wing, is what creates downforce. The presence of the wing modifies airflow over the car, resulting in slight pressure differences that need to be considered for the generation of overall downforce.
Most garden-variety wings have a constant cross section along the wingspan. Other more sophisticated wings change in both airfoil type and size in addition to a step in the wing's angle of attack (at approximately 20 to 25 per cent of the wingspan) towards the end of the span. These complex three dimensional '3D' wings can be more effective than the simple examples because the design takes into account the actual flow arriving at the wing. At both ends, the air coming off the rooftop-to-window juncture has a different angle of approach compared with the air going over the middle of the roof. By designing a wing to take into account this local airflow condition, more downforce and less drag can be achieved.
While some wings are single-element, others may have many elements. A single-element wing can generate a significant amount of downforce. However, in some forms of racing where multiple elements are allowed, downforce can be doubled or even tripled. For example, Formula One cars can have as many as four elements, contributing to the staggering amount of overall downforce produced. Multi-element designs are also used on wings with long chords (the length when you look at the cross section). Smooth airflow tends to detach from the surface of the wing when it has to travel a long distance. By using multiple elements, air from the bottom is allowed to cross over to the top and continue to flow across the topside of the next element. This minimizes flow separation over the wing just as a jet liner with flaps fully extended.
There are some devices that have been proven to augment the downforce of inverted wings. One used prominently on race car wings is called the Gurney flap, or wicker bill. The Gurney flap is usually a very small bent angle attached to the wing's trailing edge. The upward-protruding edge helps produce more downforce by increasing the vertical airflow deflection. Gurney flaps are efficient at improving downforce up to the size of approximately four per cent of the airfoil's chord length. More downforce can be generated with larger Gurney flaps, but drag increases quickly as the flap size increases. For a 10-inch wing chord, an effective Gurney flap of two-fifths of an inch could augment the wing's downforce by as much as 25 per cent.