I have stored mine in our Garage/Barn for the past 3 winters using the following:
Seafoam in the gas tank to stabilize the fuel and a full tank of gas
Small dish of Peppermint oil in the trunk and passenger compartment (to keep out the critters)
and lastly a good car cover
Has started each spring without a problem and no critters !
2008 Z4 3.0si Sport Package, Premium package, heated seats, M sport seats, xenon headlights
2002 325 xi, sport handling, rosewood trim
2005 X3 Premium package