Full on smile seems like a bit much as others are suggesting, but I have seen clear benefits from something a bit more subdued.
Your neutral expression can be more or less smily. To be slightly frowny or straight neutral usually requires a slight contraction of the jaw muscles. To be very subtly smiling you just have to relax them. You won’t look like you’re smiling, but people will notice that you’re relaxed, and then how they treat you reflects how you being relaxed makes them feel about you, which is generally for the best (unless a situation calls for you to be not relaxed!).
Full on smile seems like a bit much as others are suggesting, but I have seen clear benefits from something a bit more subdued.
Your neutral expression can be more or less smily. To be slightly frowny or straight neutral usually requires a slight contraction of the jaw muscles. To be very subtly smiling you just have to relax them. You won’t look like you’re smiling, but people will notice that you’re relaxed, and then how they treat you reflects how you being relaxed makes them feel about you, which is generally for the best (unless a situation calls for you to be not relaxed!).