Natural consequences! If she's cold, she'll zip up. If not, let her be. Not everyone has the same internal thermostat. There's a reason we joke, "I have to wear a sweater because my mom is cold!"
I disagree that this needs to be turned into a discipline issue where you establish who is boss and your child learns to do as they're told without argument. There are MANY more important issues to put your foot down over (such as being respectful, being kind, doing your chores, brushing your teeth, etc). I value a peaceful home, and believe strongly in picking my battles wisely.
In fact, dressing for the weather is one of those wonderful opportunities to allow your child a bit of independence and show her that you respect her ability to make choices for herself. I really like the suggestion by another poster to have a thermometer by the door, and teach her to read it.
The only way a child can learn to make smart choices is if they're allowed to actually make them. And sometimes she'll choose wrongly... But isn't it better that she experience that now when the worst that will happen is she'll be a bit chilly, than later in life when the consequnces may be much more severe?
Once, when my son was a toddler he refused to put his boots on. I said, "okay," and followed him out into the snow with his boots in my hand. When he started to cry, I picked him up, dried his feet off, and put his boots on. After that, he didn't argue about the boots! He's now 18 and doesn't always dress as warm as I think he should, but that's his business not mine.