There are lots of variations of the "wave". I almost always do some type of wave at oncoming two wheeled vehicles (including scooters, but not bicycles) as well as 3 wheeled vehicles like the CanAm or traditional trikes. Most return the wave, but not everyone. No big deal either way. The helmet on the ground by the rear wheel, on the left side of the bike as a sign that help is needed used to mean something, but it doesn't seem to anymore. I'm no mechanic and could not likely be much help to someone broken down, but I used to stop if I saw the helmet on the ground. Many years ago, stopping meant that you could find out what the problem was, or at least if the rider needed help or a tow, and then proceed to the nearest pay phone and call for help for the stranded rider. Since the advent of cell phones starting about 20 years ago, most everyone can call for help themselves, and maybe that's why I can't even recall the last time I saw a helmet set on the ground by the rear wheel.
Tapping on the top of the helmet, and or flashing high beams has been a pretty standard way to warm oncoming riders that they will soon encounter a police officer, often hidden from his view. I use the helmet tap method, since I can't readily flash my headlights since they are modulating by themselves, and if I flick them to low and then back to high it turns off the modulating feature.
Another sign can be pointing with the hand at the ground if there is some hazard there, giving riders coming up right behind you a warning about the dead animal, other object, or pot hole in the road. But this is obviously only used when riding in a group.
Finally, I have used a "sign" to tell car or truck drivers that are too close to me to back off, without being confrontational. I just use my left hand extended down, palm to the rear and gesturing with that hand in a "patting" motion. Usually it works, and I've never had it taken as an aggressive gesture, as would the middle finger salute. But in case any cager takes offense at my asking him to back off, I always ride armed.