I think the best advice any one can give on bomb-proofing a horse is to use your imagination. Yeah, wet saddle blankets is the best way, but at home you can pretty much do anything and the horse learn from it, no matter how silly it sounds. We've done a lot of weird things with my mare, and I think all of them helped somewhat. Once, we tied about 10 different ribbons onto the front of my horse's halter and lunged her in it; another time I rode her while dragging a blue tarp behind me on a lunge line (after getting her used to it). I've rode her with a blue tarp as my saddle, I've tied ballons to her halter, and once we even had her eat out of the back of the fourwheeler while it was running. She still is green, but she isn't near as bad as she used to be.

Like you also said, your attitude and reaction is also a big factor to the horse. If you tense up at something, the horse says,"Oohh, even the predator on my back is scared." If you're calm, your horse will know there is nothing to fear.

JMO, though. I'm still fairly new to horses, and I know this because I have great teachers.