I always thought that heat destroys magnetism, so baking would be an absolutely last resort.

Magnetic tapes suffer from self demagnetization which loses the high frequencies, and print through if they are left too long without being played. So record at the highest speed possible (so cassettes, at 1 7/8 ips, are out as a viable long term medium) on top quality tape and rewind it now and then. They only tape manufacturer that I ever had problems with was BASF - it was always breaking.

Computer back up tape is more permanent than home audio tape as it is usually high remanence but you're going to need keep the computer to play it.

I would say that the most permanent method available is the 78 or the LP. It is mechanical and if push comes to shove you can make a player

Be VERY careful about cleaning 78s. Modern cleaning solutions can dissolve the bloody things.

If you are considering hard drives then you should remember that they are also magnetic and the recordings should be backed up and checked regularly.