I'm considering taking an Item Familiar for a fifteenth-level semi-arena game, and I'm concerned about it being stolen, sundered, or specifically targeted with spells such as shatterDetect Magic and so that Divination doesn't come up with a 'smash his teeth' result, which could be extremely painful as well as inconvenient)
Make it a cursed item, many of which can't be easily removed: the Cursed Sword -2 is of special note due to its inability to be 'gotten rid of' (a clause inclusive of theft, sundering and disjunction) short of Limited Wish, Wish, or Miracle, but is pretty problematic for anyone who's not a primary spellcaster.
Make it an Eye of Horus (Dragon 325)- a magic item that can go from a lens to a tattoo and back again, removable only by the will of the user or upon the user's death.
Dip Soulknife and make your Item Familiar the Legacy Weapon Mau-Jehe, which has the unique ability to be turned into your Mind Blade. Congrats, you now have an Item familiar that can never be taken from you, never sundered, and can even be active in an AMF if you make the Will save.