For me, the relevant OS 25k map(s) are kept in the back pocket of my rucksack in case of need it (never have in past 6 yrs since I got gps). My intended route is printed out double sided on A4 at 12.5k scale and put into plastic file pockets. These are folded in 4 and kept in the thigh pocket of my trousers. My GPS is clipped to a loop on shoulder strap as my main nav aid. If I have several sheets of A4, I number them in the corners to aid identification, the used ones are put away when I have finished with them. Sometimes, at the planning stage, I will draw a ring round a feature with the number of the next sheet to use to help me move from one sheet to another.
The A4 maps are consulted frequently to give me bigger picture than gps provides e.g. how far to next turn, whats that track - where does it go?, how much further? etc