I'm guilty of keeping spray paint for too long. No matter how clean the nozzle, solids will settle out and come painting time, will clog things almost instantly, even when the can is shaken for the recommended time.
I got tired of this game, so I rigged up a crude paint shaker. I hung a cardboard box (Something sturdier would be better, but this was just an experiment.) on bungee cords fastened to the ceiling.
I put the paint cans in the box, then fastened a portable drill to the outside. Chucked in the drill was a crude eccentric made out of a bolt and a piece of scrap iron weighing about a pound.
A wrap of baling wired around the handle and trigger was used to adjust the speed. The proper speed will be different for every set-up. It needs to be adjusted for maximum movement, usually a surprisingly low RPM.
After I let things shake for about a day I got a perfect paint job with no clogs.
So many projects. So little time.