The main thing we learn from the fender skirt forum threads is that if a car wasn't designed to have them, they usually don't work too well and you have to try harder to fit them into the existing styling.
Be especially careful of the cars that really accent the rear wheels- I'm talking about the blue S2000 photoshop
as well as GMC Sonomas! Skirts tend to lengthen the car and continue body lines so I'd suggest trying to visually either lessen that Sonoma arch or make it more prominent with a semi-circular undercut. Just rambling at this point... but I think it's important to look at what the skirt will do before you go head-first into building one.
I did a few Lincoln Personal Luxury Coupe sketches a couple years back and they happen to have skirts...
Of course, the more recent
Town Car mini-project left them out.