aah i get you now. I used to get them (but i dont do "great" distance cycling just yet).
I fixed mine by drinking a cup of coffee before i leave (to ensure its not due to lack of caffine), and ensuring i drink plenty of water whilst out.
If its a cold day i'll also put a wooly hat on underneath my helmet. Instructions on hat say not to wear anything else with the hat, but i figure it cant do that much harm as long as i keep the hat fixed on nice and tight.
doesn't GET RID of it, but it has reduced it a great deal.