Selective color can be very overdone IMO. It works best when there is a distinct difference between what you want in B/W and what it looks like in B/W. A lot of times people grayscale an area of a photo that could very well be that color in actuality. That takes away from the conversion IMO. I use selective coloring VERY sparingly because a lot of times the photo would be better off staying the way it is or going full b/w...
Anywho, here is a selective color I did recently.
'51 Chevy (2) by
FLWill, on Flickr
'51 Chevy (1) by
FLWill, on Flickr
Even in these shots, whatever was converted to B/W could potentially be a similar color (or lack thereof). I wanted the emphasis to be on the pinstriping though, as the owner of the car just had it done the day before.