Vallejo make their own air-brush cleaner, this is very good with their paints and tamiyas. It disolves the dried paint very quickly. I usually clean out the brush initially with water and finish off with this cleaner.
Pay attention to the inside of the nozzle, but be very careful. This is where paint builds up which will cause most problems. I often soak this for a while and clean through with some tissue, lint free is best. or anything which will go through the small hole of the nozzle.
See their web-site for details. As Mike says, lacquer thinner will also do the job, but it is smellier and more aggressive. Alcohol on it's own does not disolve the paint as easily as the Vallejo cleaner.
Ross