You can get more use out of your paint if it's stored properly. Always store paint in a dry location where the temperature is above freezing. Replace the lid firmly and store the can upside-down to prevent air from entering the container — this will keep your paint usable longer.
When purchasing low-VOC paints, buy only what you need for the job at hand. Before you begin, measure the area carefully (height x width = total sq. ft.). One gallon of paint will cover approximately 350-400 square feet in a single coat. To calculate how much paint you'll need for your specific painting project, use the paint calculator tool.