I have no idea what to do to my car. Any recommendations?

I suggest wheel upgrades first. Nice wheels really make the car stand out. If you have 17" or 18" whels, up to a 20" wheel with tires to match. For a really aggressive look, go staggerd wheels so you have wider tires in the rear. 8.5" wide for front at 9.5" or 10" in the rear. The white strip suggestion is good depending on your car color. In my opinion this should be #2 on the list behind the wheel modification. #3 would be to lower the car. It's amazing what 1.5" lowering springs will do for the stance. These will give you an aggressive look.

BTW, this photo is my modified '06 GT. 20" staggerd wheels, lowered 2" with two white strips. Side vent window louvers work well too. Enjoy making your car your own.

Wheels make the car, however leave the racing stripes off. Lowering the car an inch would be nice. Side vent louvers won't work on a convertible.

Check out american muscle.com , they offer tons of aftermarket parts/modifications for Mustangs. If your not sure how a part will look on your car , no problem , just view the photos of customer's Mustangs with that picticular part installed. (Even in the same color as your car). Good luck and have fun.