How can I store my tent when I'm not using it for months at a time, so that it is still in good condition when I use it again?
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.
Open it up, sweep it out, let it air dry if it's damp, and pack it up and keep it dry.