I bought a Motrax "Wind Stop" neoprene helmet skirt a few weeks ago. It worked very well to reduce wind noise but it kept slipping off my helmet and started to tear along the seam, suggesting it was too small (unfortunately, it is "one size fits all"). I returned it to the Cycle Gear store and got a refund. However, it worked so well for listening to my IPod with Etymotic ER6 earphones that I wanted to give it another try (especially on my ride from Miami to Atlanta next week) so I bought another one. This time I plan to try gluing it to the helmet. Also, it seemed to fit better with the label and the wider side of the opening towards the front of the helmet instead of towards the rear, contrary to what you might expect. Anyway, I hope I can get it to stay in place because it really did seem to work. It was only $10 online from Cycle Gear, so you might want to give it a try. Someone makes a more expensive helmet skirt, like $40-$50...I can't remember the name but they had them also at the Cycle Gear store.
Additional info added: The other helmet skirt is the Quiet Rider.