what type of airbrush do you have? here are my recommendations:
1. if you're using tamiya i recommend biting the bullet and purchasing tamiya thinner. alcohol as a thinner will tend to dry the paint both on the tip of your brush and in mid air as it travells to the subject. when it hits the subject, its about 70% dried.
2. keep a q-tip damp with the appropriate thinner and gently clean the tip of the needle occasionally.
3. for fine line work, thin even further that 50/50 and lower the pressure as low as you can get it and still apply the paint in a smooth coat.
i normally spray at max. 12 psi for anything except for primer and future. for fine line work such as camo demarcation lines, i'm get down under 10 psi where my guage quits reading properly. good luck friend. later.