Layer the color or colors slowly. It is always easier to make the shading darker than it is to make it lighter again. Work your way from the darkest point to the lightest point, slowly pulling your finger off of the trigger as you go. Practice this technique by painting circles that are darkly shaded at the bottom and lightly shaded at the top.
Buy simple stencils to add basic designs to your T-shirt. To add a layer to the shirt using a stencil, cover the area of the shirt with the stencil and lightly run the airbrush over it. Too much detail can become overwhelming and might actually take away from the main design.
Instead of using a paper towel to blot texture onto the shirt, use a plastic sandwich bag. Stick your hand inside of the bag and spray paint into your palm. Blot the shirt as you would with a paper towel. The difference, though, is that a paper towel will soak up some of the ink, whereas a plastic bag will not. A plastic bag will leave your T-shirt textures with thicker, richer colors.
When applying paint to a T-shirt, use your whole upper body to move the brush, not just your wrists. Use the hand that is not your dominant hand to steady the brush while you are applying paint.