Starting below -10C is where it starts to get interesting; -20C was even more fun when I visited Sudbury, Ontario on a cold March weekend. Taxiing in I noticed that all the a/c in tiedown had on engine blankets and an electric cord leading away. I felt rather naked.
Starting began with a preheat; after a POH prime with six shots did not do the job, the mechanic took over to work the prime while the lucky apprentice pulled the prop.
Engine blankets help keep the heat in between flights, but a couple shots of prime doesn't hurt in -10C in a C-172 that has been flown in the last hour.
The most important trick is to conserve the battery and not burn out the starter motor with repeated attempts. If it doesn't fire up on the first or second try, find somebody who knows the airplane.