I know what you mean, my hair tends to be much flatter whenever I go dark - although it does stay mega shiny!
The only thing I do to add a bit more volume is backcomb the roots a bit (not much) and keep in place with a small spirtz of spray.
Oh, and I dry my hair upside down too, which seems to help :-)
You could always think about adding a bit of a curl, which would give it some volume. Obviously that depends on how long your hair is though. Mine's currently fairly short, so curls don't really work... unless I want to end up looking like Brian May!