It may seem redundant but, when spray foaming, keep your job site ventilated and do it every single time you spray. Keeping the work area ventilated will prevent unnecessary exposure to fumes and particles that can cause damage to workers. Best practice is to be aware of the airflow and have a source of entry and a way for air to flow out of the work space. Also remember to keep the area ventilated until the re-entry time specified by the foam manufacturer.
For more information on proper ventilation procedures review the EPA Spray Foam Ventilation Guidelines