tripod, no flash, no bright light, sun should hit car at angle or from side, not above from above.
use funky angles, take lots of shots, so you can later pick.
And of course camera makes big difference, so people with some kickass DSLR that has cool lenses and filters will always take better pics then those with a point-and shoot cam with normal lens.
'98 Avus Blue 320i - SOLD '06
'06 TiAg 320d - SOLD '10
'07 Montego Blue 325i - SOLD '12
'07 Z4 Coupe 3.0si - SOLD '16