I tried the method as advocated by the link Rustle kindly provided, but ended up in the pub more often than my liver could stand so did the next best thing. Copied the way professional map sellers fold their maps in their booklets!
Take your half mil and fold it in the middle along the North South axis.
Keep the map folded and fold it again in half but this time along the East West axis.
Fold the top of the map again in half along the East West axis.
Fold the bottom of the map again in half along the East West axis.
Fold the tops and bottoms again in half along the E-W axis.
You will now be able to concertina the map and go up and down finding the bit you are interested in.
If you have managed to work this out and concertina the map in you will have an approximately 66 x 14 cm folded map. To make it more portable and easy to stick into sidepockets or between seats you can of course fold this once more so that you end up with a 33 x 14 cm folded map.
I know the description is only short but I know my attention span is about 30 seconds and suspect others' may not be much longer when it comes to something as exciting as mapfolding.
Have fun!
FD