First question: Do you have electricity available?
If not, you'll need to do something with combustion heat. Think Red Dragon, or Northern Companion style.
If so, then there are a number of options you could use. At the temps you're likely to see there, I wouldn't spend as much as a Reiff or Tanis system costs. The EZ Heat systems work fine for this, are relatively cheap and are thermostatically controlled. Here's a link: http://www.e-zheatproducts.com/shop/
Get an insulated engine cover and one of those and you'll be good to go. Plug it in the night before or use a remote switch as described above.....
A lot of folks simply use a pad heater stuck to the oil pan. Be careful doing so. This is essentially what the EZ Heat system is, but with thermostatic control. The problem with a Wal Mart silicone heat pad is that they aren't thermostatically controlled and they can "burn" your oil inside the pan due to too high a heat.
These are the safest, easiest kind of pre heater....no moving parts, slow heat so thorough, easy to install.