I've tried all sorts of cleaning , boil , steam , coke, drain overnight. Frankly none of them are worth all the effort for little or no result.
I just use them and change when they either go cold or produce naff vapour.
I had one classic atomizer do 3 months heavy everyday use , never done a thing to it..worked like a charm , just went cold.
Cleaning or draining in my experience is pointless. plus , 5x 510 atomizers for £23 or 5x901 atomizers for £20 from totally wicked UK..is it even worth it financially.