I don't desensitize at home. I honestly don't find it much use - A plastic bag on a stick at home doesn't always correlate with a plastic bag blowing under their feet on the trail.
I prefer to:
1) Teaqch them HOW to respond to a scary situation. I teach them to keep on the same line and speed that I have chosen. They can look as much as they like but they need to maintain both line and speed. Mostly I flex them away from the scary object and use a leg yeild to keep their body on the same line. The more things we pass without issue, the more they trust me to tell them it is ok and believe me.
2) Same as everyone else, just get out and ride in as many different places as I can. I ride in the city, to the mall, through Maccas drivethrough, through creeks, over causeways (Sometimes flooded) through gullies, on footpaths, through sheep and cattle.
As long as your horse knows how to appropriately react to any scary stimuli, and is exposed to as much as possible, it doesn't matter if they are bombproof or not, as scary things become a non-event as they know what is expected of them - After all, that is all a horse wants!