Summary: All cushions need regular maintenance. A regular cleaning will prevent mildew, while spot cleaning will sort out stains. Store them away to prevent fading and increase the lifetime of the cushions by years.
Unless you put your cushions away regularly, they have probably spent a lot of time outside, exposed to the weather. Even when crammed away in storage, it might have already had stains. Even waterproof cushions are not completely immune to the weather. Only regular cleaning will keep your cushions ready for your next outdoor event:
Regular cleaning – Failing to clean cushions regularly, especially in the little seams, can lead to mildew. The most effective way of cleaning is to use a vacuum cleaner, with a small nozzle. Alternatively, you can just brush the cushions.
Spot cleaning – When dealing with liquid spills, do not wipe the spill. Use a dry cloth and blot as much of the stuff as possible. Oil based spills require some form dry remover like cornstarch. Then spray a cleaning solution, rinse it off, and then air dry.
Deep clean – You need a deep clean when you don’t remove spills and stains immediately. Use a solution of soapy water and a soft brush to clean up the fabric. Even allow the soapy water to soak the fabric. Rinse the fabric and air dry.
Fading – The only guaranteed way to avoid fading is to avoid storing the cushions in direct sunlight. There are fabrics which are UV resistant, but even those will fade eventually. Cover the cushions or bring them in and add years to their lifetime.