How do you prevent the valve from clogging up when using aerosol conformal coating?

I suggest you get into the habit of clearing the valve after every use. You turn the can upside-down and spray until it only sprays propellant. If you don’t do that, you run the risk of dried coating building up in the valve or actuator button. If in the button, you can switch it for another one and it should work. If in the valve, there may not be much you can do to save the can.