Allow the paint to dry thoroughly before applying an additional coat on top; bubbling sometimes occurs when paint is applied on top of a previous paint layer before that layer has completely dried.
Use a paint stir stick to mix your partially filled paint can rather than shaking it to prevent excess air from being incorporated into the paint.
Apply the paint at a slower speed, which will allow a thicker coat to be applied and may eliminate bubbles.
Apply a primer paint coat to the surface you're painting before applying regular paint; this will allow the top coats to adhere better and eliminate any air pockets.
Check the label to see that the paint thinner or reducer that you're using is recommended for the paint that you're using. Follow the directions exactly for what type of thinner to use and how much to mix in.