On many cheap scanners, the top/lid comes completely off, leaving a relatively flat surface. If you cut a couple of pieces of cardboard to about the same size as your glass, you can then lay the map onto the glass in pretty much any position without worrying about wrinkling where the paper hits the hinge. I like to use a piece of black paper on the underside of the cardboard to reduce reflections. This advice is less helpful if the lid of your scanner doesn't come off, of course (unless you're willing to invest on a super-inexpensive scanner like the CanoScan LIDE scanner, remove the hinge, and just use the top without the hinge).