You should keep your sanity up by at least 80% to make sure that you wont even start hearing voices. At lower than 50% sanity, those tribal hallucination will start spawning and acutally deals damage to you.

Things to do:

Eating well roasted meat.
Staying near fire.
Sleeping in a good quality bed.
Taking off leeches.

Things you should not do:

Eating insects.
Leaving diseases untreated, especially worms and leech.
Sleeping without shelter.