Have you tried making it into a game with putting it on? In the house, outside of the typical scenario where you put the helmet on her.
You can play with yours and she can play with hers. Maybe you two can make believe you are some kind of "professional" who has to wear a helmet... and you have to put it on to go work. Like you are playing police man or something?
Along with the game, what about sitting down and decorating the helmet. Get her stickers, stuff to glue on there... and make a game out of decorating it. You could even glue a little crown on it to make it really cool. If she decorates it and plays with it in the house, she won't be unwilling to put it on when you go ride.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"