Just pass them over with a tyre truer when you are about to use them. I don't think tyres go off to be honest, unless they've been sitting for aeons?
I've had sets of tyres sitting for 3 years in their packaging, i trued them and they were as if they were fresh out of the factory
if you want to be pedantic about storing tyres just get a jumbo zip lock bag and chuck them in.
it might sound counter intuative, but i've found spraying the tyre with metho brings up the grip again, it's especially useful if you get silicone oil on your tyres. What i think the metho might do is open up the pours of the foam compound, just a guess.
poor mans traction compound; Sun block cream
works great but you need to bake it in with tyre warmers or let them sit for days. give it a go with some old tyres, then put them in a zip lock bag once they have gone sticky.