smile at them -- they may be idiots with no real training in some cases, but they respond to polite a whole lot better.
Be polite? Sure. Smile? What have you got to smile about? You're in a hurry. You've just been made to wait in a long line, empty your pockets, remove your shoes, unpack half your carry-on luggage, and risk having your valuables stolen while they sit unattended at the end of the conveyor while some clown in blue is checking your package for heaven knows what. What you have to look forward to is putting your shoes back on without benefit of any chairs in the area, repacking your carry-on while passengers behind you are impatiently waiting for you to get out of the way, and being sure you've actually collected all your personal effects before rushing out of this island of traveler terror. Unless you're really into having strangers of the same gender fondle your privates, smiling would seem to be one of the least natural responses to the situation. Downright suspicious.
I always do my best to look bored and mildly annoyed. Since that's how I usually feel, it's not much of a stretch. Acting natural is the best approach and I can't see smiling as a natural reaction to this major annoyance.
Overall, I think your advice is pretty good, but I'd keep the smiling to an absolute minimum. At best they'll think you're weird for apparently enjoying this circus of inconvenience and annoyance. At worst they'll think you're trying to act non-threatening and pull you aside for even more security fun.