To keep my kids' feet warm while skiing, I go the low-tech route and buy a box of toe-warmers on Amazon and stick them to their feet before they step into their boots. But if you want to get more serious about keeping your feet warm in your boots, there's the battery-powered variety from Hotronic, which features a heated footbed.
Apparently it works well for most people (but not everybody). Just keep in mind that these do require a custom install, so count on paying some extra money for that. Most stores won't charge all that much (the store I was in charged $25), but you will pay retail price for the product.