For most people who want a more pleasant odor in their vehicle, the process of selecting an air freshener is simple. Simply pick up one of those tree-shaped hanging pieces of cardboard and you're done, right? While these can be effective, there are actually many choices when it comes to air fresheners, all of which have their own advantages and disadvantages.
The cardboard air fresheners are by far the most popular, and they're also very affordable and easy to find. However, you'll likely be running through a lot of these quickly if you want the scent to last for a long time. Many drivers leave these hanging up but don't notice that they stop giving off a scent pretty quickly. There are many other long-term solutions that may ultimately be more cost-effective or convenient.
In the same vein of the cardboard air fresheners are paper-based air fresheners. These have the ability to be placed almost anywhere within your car (under the seat is a popular location), so they're not an eyesore. Like the cardboard they carry the same weaknesses, however, such as a short-lasting scent. Still, they are quite cheap, so you can always buy in bulk if you don't mind constantly replacing them.
A longer-lasting option is gel-based air fresheners. These are typically sticky and can be attached easily to any surface in your vehicle. The scent may not be as strong initially as a cardboard air freshener, but these will last a long time and are completely non-intrusive. Still, the smell probably isn't strong enough to act as your main air freshener - but it makes a nice complement to other options.
The latest in air freshener technologies are products that attach to your vents. The vent stick is a plastic, clip-shaped item that easily affixes to a car's open vents. These are coated in a fragrance, which is then slowly released into the car as your vent runs. These are pretty effective, but you might not notice the impact right away - and they also require you to use your vents to work.
There are also oil-based products that attach to vents. Oil wick air fresheners are essentially plastic cases containing a liquid fragrance, along with a plastic top. The top attaches to your cars vents and have slits on them for the fragrance to pass through (which may be adjustable depending on the brand). Essentially, the oil is taken up through the wick into the plastic top, where the vent will blow it through the slits and into the car.
Oil diffusers are a recent development that will likely become more popular. Like the wick design, this comes with a liquid fragrance that attaches to a vent. However, instead of the wick and plastic top, these fresheners use a membrane, which the fragrance passes through into the vent, where it is then redistributed throughout the vehicle. These might be a bit expensive off the bat, but they last quite a long time and definitely get the job done.
Other options include canned air fresheners, which come in a cylinder that can be placed anywhere in the car, and advanced plug-ins that work using your vehicle's cigarette lighter.
Finally, if your car just happens to stink and you need it to smell good quickly, it's hard to beat aerosol air fresheners, many of which are made for use around the house. A few sprays of this will deposit a masking scent that should help hide any odors long enough for you to get through a date or some other important function. But you definitely don't want to overdo it on this, as the spray will build up over time on your seats and other surfaces, and the scent doesn't last as long as the aforementioned choices.