"But a heart can be hidden. Of course, the value of that depends on how well it has been hidden, and you cannot believe the stupidity of some of the pupils I've had. Why, one of these dolts was so mindless that he hid his heart inside the body of a lizard that was inside a cage that was on top of the head of a serpent that was on top of a tree that was guarded by lions, tigers, and scorpions! Another cretin, and may Buddha strike me if I lie, concealed his heart inside an egg that was inside a duck that was inside a basket that was inside a chest that was on an island that was in the middle of an uncharted ocean. Needless to say, both of these numbskulls were destroyed by the first half-witted heroes who came along." - The Old Man of the Mountain, Bridge of Birds by Barry Hughart.
I just thought the above quote was particularly appropriate.
There's a few tricks you can use with the imprison idea to make it work even better, should you use it.
1) Decoys. Get a bunch of kobolds, all named the same thing, and imprison them in widely varying places - other planes, dangerous and remote areas, etc. If they're not careful with their divination spells ("we want to find Bob the kobold, imprisoned by LordAO") they'll wind up haring off on wild goose chases.
2) Booby traps. Imprison a nasty surprise for people trying to free the kobolds. If they go to the site and cast freedom and say 'we wish to free Bob, who was imprisoned here for the purpose of guarding LordAO's life', they are going to be surprised when instead of getting Bob the kobold they get Bob the raging monster (whatever type you like, although I suggest something unintelligent so it's not likely to come after you). Obviously you'd have to make the circumstances of the two Bob's lives as close as you could.
3) Misdirection. Sometimes the best way is the non-magical way. Tell the people you trust - the party, an apprentice, a lover, etc - that your life is hidden in one way. Then hide it somewhere else. In the stories, it's always a lover or a daughter who betrays the secret location of the sorcerer's heart. Get drunk and brag about your precaustions once or twice if you like. Visit the location often, as if 'checking' on it. Place a magic fingerbone (or whatever you stored your life in) there - make it real magic, though, not just Nystul's magic aura. Have plenty of minions watching the location so you know who is going after your heart, because knowledge is power. If someone goes and steals the fake fingerbone, pretend like it really does have your life in it. Let them think they have a hold over you until the time is right.
4) Revenge. If you have the capability, make the fake bone (or whatever) a magic item that will do something nasty when it's broken. You might as well put the same spell on your 'real' life, too - take them with you if you finally do go.
J