Revitalize treated wood decks by pressure washing them, followed by sealing them with Thompson's Water Sealer or another wood deck treatment. Scrub synthetic decks clean with a bristle brush and mild detergent. The cost of renewing a deck and maintaining it is far less than the cost of replacing it entirely.
Lowe's sells Rustoleum Restore, which improves the look of even severely damaged decks with a unique coating that is 10 times thicker than paint. The product comes in a variety of shades to match any decor and fills in cracks of up to 1/4 inch.
If your deck is in good shape overall but starting to look a bit scuffed, apply a clear or tented sealant to protect it from water and sun damage. Treat decks once every few years if you use them a lot. Extremely high-traffic decks may need treatment as often as once a year to maintain a nice look.
Behr Deckover is available at Home Depot. This product can restore a wood or concrete deck using a textured coating that helps add traction for safety. It is available in 5-gallon buckets, enough to handle even very large deck projects, and the treatment lasts for years.