Stable humidity, out of leather sheaths, lightly oiled, and handled with clean dry hands. The three things that damage collections are moisture, skin oils and sunlight, and all three are cheap to control. Check the collection once or twice a year rather than sealing it away and forgetting it.
A room with stable temperature and humidity. Not an attic, garage or basement, all of which cycle. Silica gel packs in a closed case help in a humid climate, and they need replacing or drying out periodically to keep working.
Keep pieces out of direct sunlight, which fades handle materials and degrades leather and box printing.
Leather sheaths, which hold moisture and whose tanning chemicals are mildly acidic. Sealed plastic bags, which trap condensation against steel. Foam that is not knife-safe, some of which off-gasses and marks blades over years.
Acid-free tissue, unlined wooden cases and dedicated knife rolls are all safer.
Clean dry hands or cotton gloves, and a wipe afterwards. Skin oil is mildly acidic and leaves fingerprints that etch into a polished blade over months.
Look at everything once or twice a year. Catching a spot of surface rust early is the difference between a wipe and a permanent pit. See removing rust.