You can do the following things to prevent your tent going mildew:
1. Find a very sunny patch at your campsite with lots of dry trees or places to hang your bedding. If you have clothesline, rope or kite string, string up a clothesline between two trees in a sunny area.
2. Remove every last thing from your tent so that it's bare. Leave the tent completely unzipped.
3. Hang up the bedding, sleeping mats, pillows, even bags to dry on tree brances, large rocks or your makeshift clothesline.
4. Wait until the top of the tent is dry before unpegging it and flipping it over so the bottom gets dry. When the bottom is dry to the touch, flip it back over, peg it down and brush out any dirt or crumbs with a dustpan and brush. Add the bedding, bags and any other paraphenalia you keep in your tent when they dry.
5. Dry out your tent before you put it away for any storage by airing it out completely outsidse in your yard, if possible. The sun and fresh air produce a better smelling tent than when you dry it indoors. Only dry the tent indoors if you have no other alternative.
I bought a camping tent from http://www.lavadeals.co.uk and I use this method to store my tent.