I buy thermacare heat patches (the ones for the neck) and wrap around my ankle (with either socks or cuddle duds over to keep it in place). They last 8-10 hours usually and that's typically enough to get me through a day at work.

You could also buy some toe warmers or hand warmers and stick those into your shoes (this would be a more economical option but I prefer the heat patches because they are more comfortable).

I've been stocking up on the heat patches and am wearing those cuddle duds under my clothes already (it was 22 degrees when I left for work yesterday...UGH!!).