Clean your mattress first
You could go to town with all sorts of upholstery cleaners, deodorisers and sanitisers. But the best thing you can do for your mattress before zipping it up in a bag is give it a really good vacuum, then let it air out for as long as you can. Preferably in sunlight, in front of a window.
Choose the right mattress bag
All quality mattress bags will protect your mattress pretty well. But you have to think about how the bag will withstand all the lifting and carrying that needs to happen in between your bedroom and storage space. Cheaper plastic bags can tear, so if you want that extra protection consider a heavy duty bag like a Mattibag.
Know your enemies
Ok, so your mattress is safe and sound in a mattress bag in your storage space. All good, right? Well, almost. Your enemies are pretty easy to identify. Moisture, dust and creepy crawlies. Moisture is the big one. Your storage space should be dry. And because your mattress will naturally contain some moisture, your mattress bag should be able to breathe a little. For long term storage, get a couple of silica gel packs, and put one on either side of the mattress, inside the bag. 250g packs should be adequate.