You could also try the technique I used to use navigating a rally car - club stuff based on a 1:50000 os map. I've also seen this suggested for aviation, but can't remember where.
It involves folding the map several times in each axis, and then making a few accurate cuts. The whole lot can then be gathered in with the corner sections between folds placed together and flat - sick these to a backing card, and you can now move east/west or north/south just be turning the folds left .
It's no good for marking a route that spans a fold, but it is very neat and easy to use. For planning, I would use an un-modified map and then transfer lines across using reference points in each section.
Sticking the sections of map permenantly back to back helps as well.
Otherwise, I have done as others - cut as mich off of the edges as you can - areas I know I won't go to at all are not needed. The ledgend is not needed on the map - carry separately, etc. Having done this, folding is a lot easier. I also place several strinps of white insulating tape on an unwanted section for the flight (but on the main fold), and write all useful info here - expected frequencies, headings, QNH, etc - I don't have room for anything more in the glider !!