I just use a soft cloth (a use a few times and throw away one, like the ones you buy for washing dishes).
I wet it, and lay it on the helmet for a minute. Then I wipe away the dead bugs once they have softened.
Never needed to do anything else.
I use this to wash my visors (clear, smoked and irridium) and also my gloss finish helmet. I have never scratched or damaged either a helmet or visor by doing this.
For really heavy bug splatter I first use a soft cloth with warm soapy water, then spray and wipe with Plexus or Honda cleaner. Otherwise, I just spray and wipe with Plexus/Honda cleaner.