Make sure to thoroughly clean and air it before storing, especially for long periods of time - this will get rid of any damp and thus should help prevent mould. Aside from that, make sure to keep it dry and away from large temperature fluctuations (a loft isn't ideal for this reason.)
Best place is to store it in a dry, thermally static environment. Make sure it is well-aired, dry and any muck is removed from it before storing it. Best to check the manufacturers guidelines if there are any.