I use one of Thule's VeloVise fork-mounted trays on my rooftop Thule bars. It's sturdy and works like a charm, no muss or fuss. The bikes are always secure, even over rough roads and in terrible weather. I use a wheel carrying "pitchfork" for the front wheel. With my current setup on my Subaru Legacy sedan, I can carry three bikes and two front wheels on the roof - not bad at all.
The only catch is that some models of the VeloVise dont' work properly with disk brake tabs - not a dealbreaker for me, but could be for some.
Rocky Mounts makes a nice fork-mount tray, as well: well made, available in many colors.
Note that I would love to be able to use one of the upright carriers mentioned above, if not for the fact that most carbon-frame road bikes won't secure properly within them without risking damage to the frame.