I did some reading and chose to get one that's SNELL and DOT rated. From what I gathered the SNELL are rated for more than one impact in the same spot where as the DOT is done after one hit.
The idea being that you might hit your head in the same place more than once - the SNELL foam is harder and hence more likely to transmit an injury like a sprain or a strain or general muscle pain (all that rhymes
) but absorbs more hits the DOT only is softer (so it's nice on your head) but absorbs just the one impact in the one spot.
After you find one that is light enough, cool looking enough, comfortable enough - double check that the foam can come out for washing