They have to be carefully socialized as kits, or they can turn into cunts. One weasel trait they've retained is to bite and not let go - spend time with your kit before adoption, this behavior will manifest itself if present. Harness train early.
If you must cage, get the biggest you can afford, and try to make sure they spend 4-8 hours a day free roaming. The love to tunnel, dig, and steal. Ball pits and makeshift toys can create plenty of enrichment.
Neglected ferrets stink. Their natural body musk has a greasy component that attaches itself to things. Baths and excellent cage hygiene resolve this.
Proper diet is a must to prevent kidney, liver and adrenal disease. Spare no expense on kibble, and introduce raw chicken livers and hearts early.