It's a good idea to dedicate an hour or so of cleaning and maintenance to your outdoor furniture when you pull it out of storage at the beginning of summer. If you really want to be a go-getter, do it again at the end of summer as well. Not only will it keep your outdoor living space looking fresh and clean, it will also help to lengthen the life of your furniture.
Wood Furniture: If you haven't covered it or stored it indoors, there's a good chance it's faded, dry, and dirty. Try a mild oil-based soap and a sponge with a scrubby green side to get some of the grime off. If it's teak, make sure you've dried it thoroughly and then add some teak oil to add shine and depth to the grain of the wood.
Umbrellas: Wash with a sponge dipped in a bucket of warm water and mild laundry detergent. Use a soft bristled brush to work off tough stains and be sure to clean off bird poop as soon as you spot it. Allow the umbrella to dry with the canopy open to prevent it from mildewing.
Cushions: Mix a solution of 1 teaspoon dishwashing detergent, 1 quart warm water, and 1 tablespoon borax and let it sit on the fabric for a couple of minutes. Wipe, rinse with your hose, and allow to dry in the sun.
Aluminum: Use diluted dish soap and scrub with a sponge, being sure not to scratch the surface! To remove existing scratches, try using Soft Scrub. Rinse well with a hose.
Wrought Iron: Use diluted dishwashing detergent to wash and rinse off with a hose. If it has started to rust, sand it down and seal with spray paint, or consider having it powder coated.
Wicker/Bamboo/Rattan. Vacuum or brush off the surface with an old paint brush to remove as much dust as you possibly can. Dilute a few squirts of a mild dish soap in a bucket of warm water and wipe down the furniture with a damp rag. Rinse with a damp sponge, immediately dry with a cloth. Unpainted furniture can be rubbed with linseed oil to give it a natural, warm look. When applying oil to your furniture, be sure to wipe off all excess oil and give it a few days before sitting on it so it has a chance to dry completely.
Concrete: If you have the poured slab furniture that's been so popular recently, rent a pressure washer and blast all the grime off.