I bought a shower cap at the 99 cent store and it fits nicely over my helmet. I will invest in a more high-quality product at some point, but the shower cap does work.

I second the wool advice if it's at all chilly. My Smartwool microweight pullover works well if it's not too cold, and thrift shop wool does the job too, though I haven't found any super lightweight wool at the thrift shop (yet).