There are 3 things that slow down the ability for the car to start on cold days.
1.Oil becomes thicker in very cold conditions. One way to help this part of the process is to use synthetic oil which stays in "liquid" form longer that conventional oil.
2.Gasoline evaporates slower in colder weather. Gas has to be in vapor form in order to burn.
and last but not least...
3.The BATTERY. Batteries store chemical reactions that produce electricity. The colder the conditions, the slower the reaction inside the battery causing the starter to have less power to work with.
A starter needs a lot of juice, and by juice, I mean power. Power is equal to Voltage x Current. Since a car battery is at a constant 12 volts (approx), you need more current. There are many batteries out there that can give you this current that you need.