Most likely you have a dirty or poorly maintained booth. Proper cleanup and maintenance is key for creating flawless finishes, and also provides a safe and competent working environment for painting.
The simplest and most important way to prevent dust from entering your booth is to keep all booth doors closed. When you do open the doors to bring in the object that you are spraying, be sure that the booth is on and running in order to draw any airborne contaminants in to the exhaust filters. A high quality air filter will provide high efficiency, and the correct diffusion to prevent inconsistent airflow and turbulence. Use the filters recommended by Standard Tools for your paint booth and routinely replace them. Eventually some overspray could collect on your booth walls, floor and paint surfaces. To prevent this from happening, clean your booth surfaces frequently. Tools that can be used to clean your booth are vacuums and sponge mops. Floors and walls can be pressure washed.