Adding extra attic insulation to your home is the single most important thing you can do in your home to cut down on energy costs and reduce greenhouse gas emissions. The Department of Energy advises 19 inches in fiberglass batt insulation or 22 inches of blown insulation in your attic.
If your joist cavities are already filled, lay new insulation in long runs perpendicular to the joists. Use any leftover pieces to fill small spaces. Learn more about energy efficiency and recomended levels of insulation at Energy Star's website.