Do social games with different Sims. Try completing their Flirt and Chat stats to have a complete Sanity diamond. It should be white once filled up all the way. After you filled it up to the top, Chat and Flirt with six more Sims. Do not imitate against them. After chatting and flirting with six more Sims, your Sanity diamond will still be full. Save and quit the game, then play as the person that you were playing with before. You will notice that your Sanity will be 100% great, and will not decrease.