Prevent Clogging:

As a standard rule, whenever you are finished painting, clean the nozzle by turning the can upside down and hold down the nozzle until only the CO2 escapes, and no paint.  This cleans the inside of the cap mechanism.

Finish my wiping off the front with a rag or paper towel.

Cleaning A Clogged Nozzle:

If you have a nozzle clogged with paint, first let it soak in some paint thinner for a couple of minutes.  This will allow easy removal from all caked on surface paint with a rag or paper towel.  To further clean the insides, install the cap onto a can of WD-40, or any other aerosol cleaner / paint thinner.  This will leave the inside of the nozzle as clean as the day you purchase it.

Some users even take measures furthest by keeping nozzles in a container of paint thinner when they arn't in use. This ensures a clean nozzle everytime.