MW Note: The shorter you cut your factory antenna the worse radio reception gets, that's the trade off for looks. However you might live in a strong radio reception area and not experence noticeable loss for most popular radio stations.
In the how-to below, Steve didn't notice any reception loss with the length he cut. Some cut their antenna much shorter or opt for deleting the antenna altogether for a cleaner look, so it depends on what you want for your stang. There are other antenna options at your local parts store (windshield antenna kits, shorty matched flexi antennas, etc). Below is a free mod and free is nice when available :-)
A lot of people dislike the look of the long antenna on the Mustang. It's length seems out of place with the style of the car. There is an easy mod you can do to shorten the antenna for a better look, with no apparent affect on radio reception or performance.
The antenna and tip will get a little scuffed with this process. Give it a light touchup with a good quality spray paint before putting it back on the car.
The result is a subtle, but much cleaner look for the antenna on the car. Radio reception did not seem affected by this modification.
Before and after pics again: