If you are tall, stay away from towering hats or anything with trims that extend upwards. Choose a hat with a flat crown but go for as wide a brim as you like - you can get away with it without looking swamped. Someone who is short can try a taller hat, but keep the brim in proportion. If you're small, give fascinators a miss as they can end up looking messy.
Small, fitted hats should be reserved for those of slender frame - a big brim can be slimming but the reverse is also true. If you can't find a colour that's an exact match, opt for contrast. An almost-right colour looks glaringly wrong in photos.
And if you're likely to be posing for photos, look for a hat with a tilted brim or one that sits back on your head.
These styles are in mainstream shops now, so you won't have to hunt through milliners' stores to find them. They're easy-to-wear and add something a bit different to winter outfits. Just remember to keep your hairstyle simple, so you aren't afraid to take off your hat at the end of the day.