I've been taught two methods:
(1) The RAF method. Before flight, tear or cut out the section of the map you're going to use for that flight. Throw the rest away. Repeat for each flight - no folding necessary.
(2) A method an examiner or CFI (I forget who exactly) explained to me a few years ago:
(a) Fold the map. Somehow. Anyhow. At random.
(b) Choose an airfield roughly in the middle of the visible part of the map.
(c) Move there, and use that as your base.