The most important thing you can do is sweep your deck regularly, paying close attention to cracks and the spaces between the decking. Clean stains as soon as you see them, and trim any encroaching shrubs and bushes. Wipe under furniture and planters every once in a while. Keep an eye out for loose nails and warped boards. Basically, be vigilant.
Autumn is the best time of year for the major cleaning and sealing projects. You want temperate and dry weather -- no rain for a few days before and after is ideal. Your cleaning method is going to depend on your decking materials: Pressure washing, for example, isn't the safest idea for wood decks. Too much pressure with an inexperienced hand (especially on an already sun-damaged deck) can destroy the deck's surface. Oxygen bleach and a good old-fashioned scrub brush is an effective, and environmentally friendly, option. For composite decks, a special cleaner followed by a gentle pressure-washer rinse is the way to go.
Wait three days or so after cleaning to tackle sealing. Again, keep your decking materials in mind when choosing a stain. A synthetic resin sealant might be a good option for most wood decks, but cedar does best with a water-based epoxy. The pigments and resins in sealers are what provide UV protection for your deck, so you'll probably want to pick a solid or semi-transparent product over a clear or translucent one. Above all, make informed decisions about your deck and closely follow the instructions on any product you use -- that will make all the difference in the longevity of your deck.