You should store manga in a dry and cool place, away from direct sunlight. Also, handle them with clean hands to avoid dirt and damage.
The key to maintaining manga in good shape is to be gentle with them. Don't stack them haphazardly. Use bookends to keep them upright. And consider investing in acid-free storage containers for long-term preservation.
To keep manga in good condition, first, make sure they're placed on shelves that aren't overcrowded. Second, avoid folding or creasing the pages. And, if possible, keep them in protective sleeves.