If your cushions have removable fabric, throw them in the wash. Follow the manufacturer's instructions carefully, and allow them to air dry completely before inserting the cushion back inside. If your cushions have a non-removable cover, you'll have to use another method if you want to clean them. For cushions with non-removable fabric, consider purchasing slip covers for protection. Make sure the upholstery or cushions have been thoroughly spot-cleaned with  dish soap or laundry detergent and warm water using a soft brush, but avoid using harsh chemicals. Scrub the fabric in every area, and let the soap sink in to the cushion. You can also try tossing your cushions into a power washer if possible. It's best to rinse the cushions with a hose to dilute the chemicals. If you can't use a hose, soak a towel in water and use the damp towel to rinse the fabric. After washing, make sure that the upholstery dries completely. Wet or damp upholstery may interfere and reduce the effectiveness of protection. Furthermore, if the upholstery does not dry completely, there is a good chance that it will develop mildew. When the upholstery is dry, spray a fabric protector all over the entire surface to protect from discoloration. You can find fabric protectors at most stores that sell cleaning supplies. Check the can or bottle to make sure that the fabric protector will not damage or discolor your furniture. Reapply the fabric protector at the start of every season to minimize upholstery damage. If you must leave the furniture outside, make sure the wind doesn't blow your cushions away. An easy way to keep your cushions in place is to strap them to the furniture with Velcro. Make sure your cushions are dry before placing them into non-plastic storage bags, to ensure no mildew builds up over time. Store your cushions in a cool, dry place such as the basement, away from humid areas.

Summary:
Remove fabric. Clean the upholstery. Rinse the upholstery. Let the upholstery dry. Apply a fabric protector. Ground your cushions. Store your cushions.