less than a cap of napisan and a kettle of boiling water into every keg after its empty, put the lid on shake it all around for a few minutes, then use the pressure built up to clean the beer/out line and the in/co2.
Clean and shiny, smells clean too.
I have had what RVJ says below as well, particularly in PET, even just a quick shake with napisan and hot tap water gets rid of it.
"Wow, I guess you're lucky then! Regardless of how many times or how thoroughly I rinse freshly- emptied bottles in plain water, a dusty residue builds up on the inside of my glass and PET bottles after cycling through a few batches, while like the OP, my kegs have the same sort of strife before too long as well. I found that the residue seemed to lead to random bottle infection (and I wasn't confident that the containers were sanitary when refilling either) so now I periodically soak all packaging containers in a strong perc solution or PBW if I'm feeling enthusiastic. Since changing to the routine perc soak, the random bottle infections are nearly nil although in all fairness now that I keg beer I do bottle less."