When you say 'protect' are you meaning from the elements? From sun exposure, or from soil?

To protect against harsh sun exposure, it is best to remove cushions when not using them as fabrics can fade, especially darker ones.

To protect from dirt consider using lawn furniture covers when not in use. (These could also be used to protect against the sun- but some fading could still occur)

To clean cushions, use a mild detergent/water and a coarse brush to remove dirt and then let air dry outside.