Engine heaters, listed by effectiveness:

1. In-block water heater
2. Tank-style water heater (circulates water)
3. Hose-style water heater
4. Magnetic heater on side of block
5. Heated garage
6. Dipstick oil heater
7. Bonfire under engine (toss-up with #6)

You want to heat the water as this warms the whole engine rather than just the pan. The closer the heater to the core of the engine the better.