Use a headband
I have baby fine hair that doesn't stay pulled up in a ponytail, and if I leave it free under the helmet I get the worst helmet hair imaginable.
I purchased these plastic headbands that have little comb like fingers at the top and sides which serve two purposes.
1. Hair stays out of face, no stray strands fall down into my eyes or whip me in the face.
2. No flat hair at my crown when I remove the helmet.
Even if it becomes slightly disheveled, I can just take the headband off and put it back on and be just fine. The headbands are so thin, they fit under the helmet comfortably, even with my eyeglasses too!