http://www.youtube.com/watch?v=sLvDXdJwwhE "The player who pulled off the spectacular play you see above is Wake Forest Rolesville (N.C.) High returner Petey Williams never really got up to top speed and didn't make it to the end zone during his team's season-ending playoff loss to Leesville Road (N.C.) High. Still, he might have pulled off one of the returns of the year. Prep Rally has never, ever seen anything like Williams' back-breaking move."
I thought it was the opposite, sitting on concrete dries out the tire creating a hard flat spot, so its recommend to use stands, or put a barrier between the tire and the ground like a piece of carpet.
Park on a carpet patch, I believe some carpet stores will give away like 2x2 squares that they have from old samples and scrap off the roll. Hook up battery tender, like $30 for the small one at IP, cover with breathable cover