How do you determine if you are using the right amount of air pressure?I second TazMan's recommendation of Vallejo Air. Thinning a little with Vallejo Air thinner seems to improve adhesion (more binders in the thinner, I think), but don't overthin. The Vallejo primers are good too. I tend to protect the finish with their acrylic varnishes which airbrush well, but usually need an higher air pressure than the colours.
How do you determine if you are using the right amount of air pressure?
Manufacturers of airbrushes and paints usually make recommendations, Vallejo say 20/25 psi for the air range. However, much depends on variable factors such as the airbrush itself, the distance of the work from the airbrush, how fine a line you're attempting, the surface you're spraying onto etc (Sorry!that doesn't sound helpful!) The only reliable guide is to test spray, starting at the recommended pressure and adjusting it until you get the results you want; it's a fine balance because you don't want the paint particles to land as a dusting and conversely, you don't want pools or runs by blasting too much paint onto the model.
Usually, don't try to get coverage in a single coat, keep the air brush moving and build up the coverage with multiple light passes. There are lots of online videos, but actually practising on old models, plastic cups etc will help you progress. Enjoy yourself !