Spray paint is a great solution for lots of small projects around the house, but it can be difficult to use the cans more than once because the nozzles clog with paint. The easiest solution is to clean the tip as soon as you finish painting by inverting the can and pressing the button until the resulting spray turns clear.
If its too late for some of your spray cans, try removing the spray tip from the can and soaking it overnight in mineral spirits or laquer thinner. Clean the softened paint from the tip using the tiny piece of wire from the inside of a twist tie (commonly found on bags of bread). Your spray can is as good as new.