found this on a french website that I can't seem to track down anymore (so not my idea)
simple but effective ;
for stirring paint (I tried this with the vallejo kind of squeeze bottles) just insert one 5 mm inox nut
(a nut is what a bolt is screwed into) and shake the hell out of it
I find it works real well
had one try with a metallic paint in the older plastic pots from gw as well
a box of 100 of those nuts costed me € (or $) 3.20, so will last me a lifetime
for all you lazy not so good english speaking persons ; this is a nut :
on the other hand, I'd appreciate ideas on Vallejo paint : Model colour or Game colour ?
cheers