When I don't need it anymore, I discard the contents of the cap, wipe it clean and screw it back. This way, the ink stays clean and I don't feel I waste much of it. Sometimes when a vial almost completely runs out -- because I have in fact put the ink in a pen shocker -- I'll add water to the vial itself and mark it for 'paint'. This doesn't always prevent me putting it in a pen anyway, but hey.
geekery, drawing and then some