The main thing is to make sure it is dry and aired before you put it into storage. If it is at all damp, mildew will form.
Store it in its bag, in a dry place where mice etc can't get at it.
Mud and dirt can generally be brushed off when the canvas is dry, Scrub with water if needs be, but don't use detergents as they contain wetting agents which are bad for any proofing the canvas may have had. If you see kids blowing bubbles, keep them away from your canvas; bubbles are floating blobs of detergent.
A little methylated spirit will deal with oil/grease marks.