I tried some DIY earplugs out of a piece of rag on a 40+ MPH descent that drove me nuts with wind noise. I didn't notice much of a difference, if any.

I think the best solution is a combination of earplugs (not so well-insulating, because you still wanna hear what's happening around you) and a headband or something that will cover your ears and streamline your head a bit. I found that the ear pavilion generates wind turbulence which accounts for the mid- and high-freq sounds, so a streamlining of the area by covering it up with some fabric eliminates a lot of the wind noise with minimal impact to your ability to hear incoming traffic and stuff. However, the low-freq, higher pressure sound still passes through light fabric (that "boomy" sound) so some cheap earplugs like purpose made foam ones or even cotton balls should further reduce wind noise. This would be the discrete, as in "not so weird-looking" solution, if you have a headband that works for you (they don't really do it for me).

Ideally you would have a hard shell around the forward side of each ear that practically eliminates wind flow over your ears, thus eliminating wind noise. Try making one of your hands into a cup over the ear (like when you imitate a fish) while riding fast and witness the magical disappearance of all wind noise. At the moment I'm trying to design something that works like this but isn't goofy looking.

LE: Just checked the Slipstreamz link. Looks like a nice, discreet solution for commuting or city riding, but I'm pretty sure it's near useless at speeds above 20-25MPH, especially if you have even mild headwind.