When the flame heater doesn't fire you are filling the intake manifold with raw diesel, which could lead to hydrostatic lock. Be careful when flipping that switch.
Despite all the warnings (PS Magazine), ether can be used, but use it only when cranking and in small "puffs" and DON'T use the flame heater at the same time.
Nothing beats fresh batteries, the engine must crank rapidly to start when it's really cold and I don't touch the "gas" pedal when starting.
After it starts. let it run for a while at idle to build up oil pressure etc. The turbo has no oil supply until the oil pressure is up. Multigrade oil will help in cold weather, although I'm sticking to 30 weight since it doesn't get below zero in my area very often.
Other items to keep in order include clean filters (fuel and air), drained primary filter and fresh diesel fuel. Water in the lines can freeze.
If the weather stays cold for days and you need to use your truck, start and run it for 20 minutes or so, every 4-6 hours around the clock and it will start very easily every time at temps way below zero.