a garter belt is a fun fix!

this is gonna be hard to explain in words, or maybe not:

take a pair of stretch cotton/lycra blend leggings (most walmart or target brand leggings)
cut off the ankle of the legging. it is not usually elastic, but rather a relatively small piece of cotton lycra with stitching around it.
take this small ring of legging and whip-stitch by hand onto the inside upper edge of the slipping leggy (the part that sits around your upper calf/knee area)
this should stop slippage

alternative: get a strip of metallic spandex (very grippy, can be found at syfabrics.com) and sew it around the inside edge as described above WITH THE METALLIC SIDE TOWARD WEARER'S SKIN....

Use what they use for thigh-high stockings. There's a rubber ribbon that you can buy at the fabric store. You attach it to your leg warmers with a long loose zigzag stitch, on the upper inside so that it grips the skin.

A friend of mine makes industrial-strength garters with mitten clips. For furry legwarmers that end below the knee, I do something really super-simple: I take a shoestring (actually parachute cord, but it looks like a shoestring) that matches the fur and tie it tight right below my knee. It was supposed to be a temporary solution for the first pair I made until I could add elastic, but it worked so well, that's just what I do now. It's comfy, it rarely falls down because there's no stretch, and it's easy.