^Plastic sheet keeps moisture away from the car's underside, possibly preventing any undue corrosion. I park on plastic too. Buy at Home Depot- painter tarp 3-4 mil.

Yes, use a fuel stabilizer, like Sta-Bil. Run-in the product a few minutes, prior to shut down- so it gets into the entire fuel system. Make the tank as full-full as possible, as this keeps out air and potential condensation/moisture.

I move my car a few inches, at the beginning of each month. One month forward, next month back. Then I do this weekely, the last month of hibernation. Very easy to do. Necessary.....? I don't have any flat spots, so I'll continue to do so. Some tire brands are VERY prone to FS. Mine, are Michellin, and not known to have this issue. But, like I said easy to do, so I'll continue.

As Falvor said, plastic is good.
Unfortunately I cannot store my vehicle at home. It is stored at a commercial facility that stores hundreds of cars/campers/boats, etc.
I never notice flat-spotting after storage with 60psi (obviously reduced when taken from storage); but I do notice flat-spotting for a couple of km if I don't use the car for a week in the summer.
I stored my Miata before Boxster and in 10 years of winter storage have not had any issues.

Thanks Andy and Flavor for the plastic sheet idea. Will add to my routine too! Bought some stabilizer as well and will add before last fill up this weekend. Will be a sad weekend...but will look forward to spring like never before!!!

If you disconnect the battery, make sure the window glass will clear the soft top rail prior to removing battery power, such as leaving both doors open when disconnecting the battery. Seal the open space between the glass and top with foam, cloth or tape.

It's done...put away for the winter, resting on a sheet of plastic. Didn't disconnect the battery, as I bought a trickle charger (cigarette lighter)from Porsche. As for the tires, my Michelin Pilot Sport PS2's say max. pressure of 51 psi's, so that's where they are now. Can't wait for spring!!!!!

Good Overview of Storing your Boxster!!
 

I found a great overview for storing your Boxster. Just a great list.

If you live in an inclement weather area, winter storage of a Porsche Boxster is a major part of your yearly maintenance schedule. You need to learn the correct methods to Winterize Your Porsche Boxster.

-Callahan-