Leaky Boots = Wet Feet
Winter boots are supposed to keep your feet warm and dry in northern climates. Unfortunately, small cracks near where the upper part of the shoe meets the sole can allow snow and slush to enter and melt, leaving you with damp socks and cold wet feet.
For leather boots, treating the outside with a good coat of mink oil will make them water-resistant AND soften the leather at the same time without causing discoloration. After the oil has set into the leather, use a water repellency spray, focused liberally around the seams of the boot. You should repeat this treatment every 3 weeks or so. Give your boots an extra boost by spraying their inside as well.
For all other types of materials, a water repellant spray for your kind of boot should help ward off leaks. You may want to reinforce the seams with some of the same compound used for repairing sole cracks.