As far as I know, the PG keeps them from drying up very quickly. I've had leftover
juice sitting in a condom for a week (not a total slob, just forgot it was there
) and it was still the way I left it. The flavor on the other hand...yeah, you might wanna do what Imagine said.
Cleaning them doesn't always get the crud off the heating coil. One way to tell is if it gets really hot after a few minutes of use, even though there isn't much vapor action - when that happens, most likely the coil is still encrusted with crud, preventing the juice from getting to it. That crud is really, really stubborn too. Once it gets that way, you'd have to decide whether you wanna keep trying to clean it, or move on to another one...