To maintain your exterior wood deck, it is best to treat it usually every two years depending upon what is currently on the surface.
Deck stains are usually classified by transparent (showing most of the wood-grain, slight color), semi-transparent (a little more color, still shows wood-grain), semi-solid (more color, less wood-grain), and solid (totally opaque, no wood-grain, looks like paint (but don't use paint).
When you have a new deck, usually you want to see the wood-grain, so going with a transparent or semi-transparent stain is best to add protection and still enjoy the beauty of the wood.
As your deck ages, then you may want more color to hide imperfections, and more color to protect the wood. The more opacity in the stain, the more protection for the wood. A solid color stain gives you the most protection from the elements, because it is covered completely. These products come in oil-based and water-based; it is important to know what is currently on the surface before re-coating with a product.
Pressure-washing is key to prepare the deck for the new stain. Sometimes additional cleaning products are required to remove mold and mildew from the surface. But be careful of the chemicals used because they may hurt the surrounding shrubbery.
Read the instructions, and always cover the shrubs. Every project is as good as its prep. This is not as rewarding as applying the color; however, it will make a difference in the long run for durability and staying power.
Make sure when choosing the brand of stain that is has a stain sealer; some of the newer products have a clear coat that can be reapplied every year. As with many products, constant research and development is being done to make these products perform better and last longer – as always, do your research.
Regarding color choices, it's important to take into account the exterior house color. You want to complement it or, depending on the color of the house, sometimes matching the house color to the deck gives a look of extending the house.
Staining the railing and spindles in a contrasting color like white (solid-color stain, not paint), gives a high-end look. The red/brown tones and the wood-look brown tones are neutral and a safe choice for resale. Be careful with going too orange on an exterior deck; it can really stand out, and that may not be a good look.
Sprucing up what you have, if you cannot sell right now, is always a good move. You get to enjoy it, and it could be that selling point down the road.