M.F.
Oh boy! this brings back memories of hard fought battles!!
tip #1 buy a fleece hat. they are way more comfie. The knit hats are scratchy.
tip #2 make sure its not too tight. Take your child to the store and try it on her.
tip #3 dont get the kind that velcro under her chin. They will make her too hot and she'll want to remove it.
tip #4 practice in the house for a little while untill she gets used to the feeling of the hat on her head.
provide a distracting activity (like painting? playdoh? cooking?) or anything you don't let her do on a daily basis so she won't think about the hat being on.
tip #5 keep putting the hat back on and say things like "we can't play outside without a hat".
tip #6 don't "make" her wear a hat in the car/store because they are heated and she'll be too warm.
hope these help ;)