I find a Classic cotton bicycle cap works most days for my 15 mi. commute. I buy them on sale for $5-$6 (Nashbar usually).

During heat waves (just coming off a week of 90+ temps w/ several days 100+ here in L. A.) l would grab a Headsweats *Dew Rag*. Though I now have gone into Summer Mode and gotten the fall/winter accumulation buzzed off. I'm also trying out a couple of cycling caps made from a *wicking fabric* like the fabric in the Headsweats.

An Important Plus of using something between your Sweaty, Greasy hair/scalp and the helmet is reducing/eliminating the development of the dreaded HELMETFUNKY pad syndrome!!