Downtown parking garages can give a good urban backdrop. I personally feel like urban suits sedans and european cars best, but I still shot mine near the leveque tower.
http://i72.photobucket.com/albums/i169/1fynzx/IMG_1048.jpg
http://i72.photobucket.com/albums/i169/1fynzx/IMG_1076.jpg
For sports cars, though, I think a great countryside works best, especially american cars. Seems to just convey the open air, warm summer feeling that goes along with the cars(although right now this isn't really an option )
http://i72.photobucket.com/albums/i169/1fynzx/IMG_1162.jpg
And using a rolling rig
http://i72.photobucket.com/albums/i169/1fynzx/IMG_1213.jpg
Good god spring cant come soon enough....