Unfortunately the ultrasonic method has been tried already, If I remember right, the Emperor of Ice Cream tried it last year some time with little success although I don't know if he used coke in it

Personally I have also given up trying to clean them to extend their life, I have tried boiling, vinegar bath and steam from espresso machine. If they cease to give good vapour I just replace them. This week has been particularly bad for me, 2 janty sudden deaths (1 fogger and 1 dura) and two generic classics just not giving out anymore (about 3 weeks old)
I think this is one aspect of vaping life that you just have to come to terms with, saying that sometimes you get a nice surprise - I still have a 901 atomiser that is over 4 months old and still performing well and I have no idea why!