I have a couple of silk tops I picked up in the charity shop that aren’t to my taste but make excellent light but toasty underwear.
I also have a woolly band kidney-warmer that I can wear under a jumper for extra warmth without feeling restricted by another layer.
Wearing a hat helps keep warm. I quite like tucking a hot water bottle behind my back when I’m sitting working.
Close curtains and blinds at dusk, and shut doors. Block up chimneys when fire isn’t on.
Many things work just as well cut in half, or using half the amount.
I’m a fan of rubbish meals (more because I’m perverse than actual frugality) e.g. soup made from pasta/rice water and the off cuts of veg like mushroom stalks or asparagus trimmings (that are perfectly good but cut off so all the stalks are same size). Stock thrills me because it comes from nothing all, and I love nettles in stir fries.
A small amount of leftovers added to something else can make a nice meal - pasta bulks out an omelette nicely, a spoon of chili as a topping on a baked potato, etc.
Chop a mars bar in pieces, and pop in the freezer. It takes much longer to eat than scoffing a bar at room temperature.
Sliced bananas frozen and then whizzed through the blender makes ice cream. Great way to use up very ripe bananas, dc happy to get ice cream and mum happy to feed them fruit 
With kids clothes, knee length dresses become mini dresses, long leggings become cropped leggings and out-grown leggings can be chopped to make knee bands to extend the life of current leggings! Trousers become shorts, long sleeve tops can become t-shirts.
If you’re handy with a needle, buy bigger sizes in a nice girl’s dress and when the first one is out grown, cut it down into a doll’s dress. Dolly and me type outfits are massively pricey and you’ll have a matching set for years for nothing at all.