Whenever I get a new OS map I...
Remove the cover;
Refold it in half, half again, etc. (normally 4 folds in all); that way you can use both the original and the new folds so the map fits the map case with your area of interest viewable on one or both sides of the map case without needing to open, unfold, refold, and close the map itself on a windy and / or rainy day;
Highlight the Eastings in one colour and the Northings in another;
Highlight the 100KM Square Id;
Write the map number in felt pen in each corner of both sides of the map so I can generally get the one I want easily from the shelf;
Pencil in the Magnetic Variation on the part of the map I am going to use "today".
I also occasionally add a small Post-It note at a useful place if I need any additional notes, grid refs. etc. for the trip.
The work of a few minutes and makes the map easier to use, particularly so if you are using a GPS along with it either for navigation or planning waypoints for a route.
Pimp Your Map!