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
shatter. This inspired me to compile a list of ways to protect an Item Familiar- or any other crucial magic item, like a wizard's spellbook or any adventuring character's Handy Haversack- from such hazards. If it matters, I have access to fifteenth-level clerical casting, the Magic domain spells and its power, first-level wizard casting, eighth-level psion casting, and binding of up to third-level vestiges, but I'm not just looking for info that will be relevant to me.
I'm aware of the 'losing an Item Familiar/spellbook/crucial item will completely cripple your character, ergo either a GM is a dick and will no matter what or isn't and won't no matter what' line of argument, but I still think that looking for ways to protect your gear is worthwhile for intermediate cases like arena games and GMs of mild to moderate dickery.
This list is obviously very incomplete, and would benefit a good deal from your contributions.

SUNDER/DISPEL/SPELL PROTECTION:
Enhancement bonuses: Each +1 of enhancement bonus adds 2 to a weapon’s or shield’s hardness and +10 to its hit points. Pretty straightforward. (And anything can be an improvised weapon!)
Special materials: Adamantine (high Hardness, core), Aururum (weapons, armor, or shields made of it can be simply put back together as a full-round action if sundered: BoED), Riverine (immune to all damage and most spells: special and total vulnerability to Disintegrate, Rods of Cancellation, Spheres of Annihilation, and MKDJ, ) (Stormwrack)
Spells: Hardening (+1/2 CL Hardness: Wiz/Sorc 6, Artifice 7),
Magic Items: Crystals of Adamant Armor/Weaponry, Legacy item special sunder rules,
Anti-dispel: CL boosts for crafters, Ring of Counterspelling (common Dispel spells),
THEFT PROTECTION:
Magic Aura (core spell: make an item look like a less attractive target and create decoys from mundane items)
Sepia Snake Sigil (core spell: a classic spellbook protector)
Fire Trap (core spell: a classic backpack/Heward's protector)
Secret Chest (core spell: keeping your equipment in one nightly keeps it from thieves but also keeps it from being used in case of ambush)
Instant Summons (core spell: a pretty good start at a complete solution, but for the material component and the fact that you only know who and where the thief is if someone else has claimed ownership of the item)
Astral Projection (core spell: make backups of everything! More TO than PO)
Armor and Equipment Traps (BoVD)
Indisputable Possession (1st-level artificer infusion, 10 min/level: call an item to hand as a move action if you realize it's missing within one minute)
Called armor/shield quality (MiC: armor is teleported to you and donned as a standard action if you're on the same plane...if it hasn't been worn by someone else in the interim)
ITEM FAMILIAR SPECIFIC:
Make it a Tooth of Dahlver Nar (ToM)- who sunders, steals or dispels teeth? (Combine with Magic Aura and possibly Obscure Object to ensure that your mouth doesn't glow to
Detect 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.