I am to assume you are using emamel paint? I would use Testers paint thinner. I would then mix the paint at about 20% thinner and 80% paint. Mix well. Check the paint, it should have the consistancy of milk. Add more thinner if needed. Try spraying some and see if it coats well and lays on smooth. If it is too flat or even rough, you might have too much thinner and it is drying before it gets to the surface. To clean, pour out the paint, wipe the cup/ jar clean and spray about half a cup/ jar of airbrush cleaner through it. When done with the paint session shoot about half a cup/ jar of lacquer thinner through it and then do a normal cleaning of your airbrush.
If you are using a true lacquer paint then you will need to get additional advice. The automotive guys have lacquer thinner and lacquer reducer.
Msgt USMC Ret M48, M60A1, M1A1