The nozzles on spray paint cans are precision-engineered and the orifices are typically laser-drilled for accuracy and spray pattern. Once used, even though the can has been inverted to clean the nozzles, there is a film of paint left inside. This film can affect the spray pattern and atomization in subsequent applications.
To keep nozzles clean or to renew a clogged nozzle, soak it in lacquer thinner for a few hours. Then put it back on the can and invert to spray out the solvent. Keep a few extra, used, nozzles around as backups. Sometimes a new can of paint won't spray well and the replacement of a nozzle frequently fixes the problem.
When using masking tape, remove it before the paint cures. That is, remove the tape within a few hours of painting to minimize the "rough edge" effects common to masking. If a sharp edge is absolutely necessary, gently run a new razor blade or X-Acto knife along the edge of the tape before removing. Special tapes specifically designed for this application are sold in auto paint stores.