Autumn's fantastic tip for helping toddlers get accustomed to wearing a helmet:
My husband and I are both snowboard instructors and have recently started teaching our son, Noah (18 mos) to ski. [WOW. — Ed. (who didn't ski till high school)] We wanted him to be comfortable with wearing a helmet long before he hit the slopes, so in October, we stuck one of our old helmets in his toy box. Over time, he started thinking it was great fun to wear it while playing, since it appeared to be just another toy. By the time January rolled around, he thought nothing of sticking his own big plastic noggin-protector on and it didn't interfere with all the other new equipment involved with the sport.
I think other parents might want to give this a try, even for spring bicycle helmet season. Just make sure the helmet that gets knocked around for "practice" is not the one you depend on for real protection. Any helmet that takes a big hit should get retired, much like a car seat post-accident. Another acclimation step we used was to try the ski boots alone indoors one day before adding the skis themselves. That's not an option for everyone if you plan to rent, but it works well if you can borrow or buy used.
Related: How to teach a kid to ride a bike