Three enemies, in order of severity
Light
It does the most damage and it is the only fully irreversible one: faded pigment does not come back. The ultraviolet component is the worst, but visible light too, accumulated over years, bleaches.
The countermeasures are simple: never direct sun, UV-filtering glazing in bright rooms, and — if you own several works — rotate them. A work that spends half the year in a drawer lasts twice as long.
Humidity
The safe range is 45-55% relative humidity, at ordinary lived-in room temperature. What matters more than the absolute figure is stability: paper absorbs and releases moisture, expanding and contracting, and it is the repeated swings that produce cockling and permanent creases.
Above 60% comes foxing, the small round brown spots, and in the worst cases mould. Bathrooms and kitchens are the worst rooms in a house. A cold exterior wall is the worst spot in a room.
Materials in contact
Ordinary card, adhesive tape, glue and untreated wood release acids that migrate into the paper and yellow it, starting at the point of contact. Everything touching the work must be acid free: conservation-grade mat board, Japanese-paper hinges or corners, and no adhesive tape, ever.
When the work is not hung
Store it flat, not rolled, in a rigid portfolio, with a sheet of neutral tissue between works. A tube is fine for shipping but not for storage: paper takes a memory of the curve, and after months it will not lie flat again without intervention.
The right place is a closed cabinet in a lived-in room. Not the cellar, not the loft: those are the two spaces with the most violent humidity swings in a house.
Cleaning
On the glazing: a barely damp microfibre cloth, never spray directly onto the frame — the liquid runs into the rebate and reaches the paper. On the work itself: nothing. If there is a stain or a ripple that worries you, a conservator can resolve it; a home attempt usually fixes it permanently.