Helmet liner using shelf-liner material
I wanted to get a little more warmth with my Bell and Giro bike helmets and I came up with a reasonable and inexpensive way to extend the comfort zone for the well-vented helmets. Just get some shelf-liner material, the thick cushioned type that is black, and cut out a pattern for the inside of your helmet. The pattern is basically a round shape with a couple pie-shaped wedges cut out of it for the straps and to conform to your head. It allows ventilation and doesn't give me brain/skull-freeze that I get with a skull cap. You may need to loosen the rear/chin straps a bit, but it does the job pretty well for 30s and 40s temperatures. Do be careful about making a template or cutting the liner material directly inside the helmet to avoid cutting your helmet straps! You may want to tape them down if you do that.
I think that by keeping my head warm I have improved keeping my hands and feet a bit warmer (the toughest to keep warm). Just thought I would pass this idea along since it's so easy to do and is relatively inconspicuous if you prefer not to have a helmet cozy or cover.