Understand the shape of your head.

I didn't know myself ....I cannot wear any Nutcase helmet style because those tend to be round helmets.

I wear my helmet pleasantly snug but it's not pressing me anywhere. I don't really feel much pressure anywhere on my head. I don't really notice my helmet on my head. Not even in the winter, when I readjust for a headband. (That's all I wear underneath helmet.) I just get ...slight wave helmet hair dent. I don't get any dents on my forehead.

There are certain sunglasses which has an arm that's too thick at the sides that hurts my head with helmet strapped on, ,so I'm picky about sunglass arms too and their fit.