Obviously I don't know how to stay warm in winter since I have already been sick ^^ But theoretically I should know a little something about it.
- Put on as many layers as possible
I always wear an under shirt, sometimes even an long sleeved one. I put on tights, socks and another pair of socks. I prefer JSKs over OPs because a blouse under the JSK adds another layer but you could also put on a blouse under an OP. Just layer up ^^
- Never forget your scarf or gloves
You could switch the scarf to a fur collar, it doesn't matter as long as you have something to keep your neck warm.
I prefer leather gloves or knitted ones, only because I think they are warmer. They don't even have to be real leather. But this is just my personal preference.
- Get a pair of warm bloomers
I have a pair of bloomers made out of fleece and it's so super worm and comfy. You want to keep your feet and your bottom warm to prevent a cystitis (==> 3 pair of socks ^^).
- Keep your head warm
You lose most of your body heat over your head so you want to put on a hat or something. I have berets in all shapes and sizes. If you don't like that, maybe you like earmuffs?
- Include fashion items in your outfit that keep you warm
Like a cape over your coat would be another layer or a muff to put your hands in.
- Use "technology"
Have you ever heard of pocket hand warmers? You bend a metal part in a gel cushion and the cushion starts to get hot and hard (that sounds wrong XD). After it cooled down and it's completely hardened, you boil it and it's reusable! YAY!
- Choose thicker and warmer material
For example, a dress made out of velvet is much warmer than a dress made out of plain cotton (even though in most of the cases velvet is also made out of cotton... it's complicated ^^). Wool would also be warmer than cotton. There are some brand dresses made out of wool. Innocent world has some, also Metamorphose temps de fille.
WOW, that's quiet a long list. I never thought I could come up with so many. I hope this was helpful at all. If you have any other tips, let me know in the comments below!