Probably get many will not think this is a good idea, but those very cheap rubber pads you get (like small domes) a few mm across that are sticky back.
Just stick one in each corner, and then you put it down it will be sitting 1 or 2mm off the surface on the rubber domes and not on the glass.
Like this, but of course, much smaller: http://cgi.ebay.co.uk/SLF-ADHESIVE-...ltDomain_3&hash=item5d28e97e4d#ht_3138wt_1199
Just stick one in each corner, and then you put it down it will be sitting 1 or 2mm off the surface on the rubber domes and not on the glass.
Like this, but of course, much smaller: http://cgi.ebay.co.uk/SLF-ADHESIVE-...ltDomain_3&hash=item5d28e97e4d#ht_3138wt_1199