Vacuum dust and dirt off the furniture, paying close attention to seams and folds in the fabric.
Clean the furniture with mild soap and water. Do not scrub, instead use a towel to wipe down the fabric with soapy water. Rinse the soap off with a hose, and let the furniture dry completely.
Spray the furniture with a weatherproofing spray, making sure the seams are well-coated. These sprays can be found in home goods stores or in sailing supply stores. Be sure the spray you select is compatible with your furniture's fabric.
Store the furniture inside to protect it from moisture while the spray dries. Let the spray dry completely before using your furniture.