Mrs Vanvlak suggested a tip for storing spray cans she'd heard from an art shop - I had previously complained and asked on the forum about spray can storage, as the nozzles tended to clog up - and it's a simple one: store them upside down, nozzle end downwards (with the top on of course!). Not certain how it works - I'd appreciate that the fluid descending would prevent the exit tube from congealing, but I'd had the impression it was the nozzle that was the problem, not the tin, and the nozzle should not have spray paint in it when it's not pushed.
I must have been wrong, because I even turned some old tins over (and replaced their badly clogged nozzles), and they worked again too.
Of course it's also key to shake the stuff well (2 minutes at least) before use, and spray in short bursts instead of long sprays.