Stain and seal your deck every 3-4 years. This can help to protect the wood from moisture that may otherwise lead to the deck needing to be replaced entirely.
Before making any repairs to the deck, remove dirt and wood fibers with a pressure washer.
If a nail that has worked itself loose from a board, remove the nail. Use a screw to reattach the board.
Pressure-treated lumber resists insects and decay, it's still vulnerable to moisture and the sun's rays. To preserve it without changing the color, use clear wood preservative that contains a UV protector, which will bring new life to the surface while protecting it from the elements. If you want to add color, use an exterior stain first. Always use the semi-transparent for the decking area, but try a solid color if you want to highlight railings or banisters.