A blanket around the cowl, with the heat inserted under the engine using scat hoses, over the course of several hours, works well. I've seen people burn the paint off cowling on light airplanes by attempting to put too much heat in too fast, especially through the inlets at the top of the cylinders (where it doesn't do much good.
Heat rises, of course, and allowing the engine to warm from the bottom, up, means that the entire engine eventually gets warmed. Manufacturer recommendation is to allow at least 5 hours for proper preheat, when using forced air.
I have also seen pilots cause leaks or damage seals by starting in cold weather and cycling a propeller full of cold, congealed oil.
Pre-oilers are good investments for those operating in cold weather, but rearlly are a wise investment for those operating in any kind of weather. Interestingly, they're not seen very often, any more. A preoiler circulates oil before the engine start, throughout the engine, to prelubricate (and warm) the engine. They're also useful for post flight shutdown lubrication and cooling.