Many years ago, when I first started working, I was given this little nugget of advice, and it’s stuck with me ever since.
Even when you are speaking on the phone, smile. Your smile will be conveyed through the phone and the person speaking to you will hear it and respond in kind.
I’ve noticed, the happier the people around me are, the happier I am.
What really drove (haha) this point home, is my regular morning bus driver.
Every single morning when I get on the bus, he greets each and every passenger with a boker tov (good morning) and nisiyah tov (good travels). On a Sunday morning he adds the greeting sh’vua tov (good week). On a Thursday he wishes everyone a Shabbat shalom before they get off the bus. At the beginning of the month he will add chodesh tov (good month) too. He is always smiling, laughing, having a conversation with the passengers seated near him. He never gets upset at the traffic/other drivers and he plays an awesomely eclectic collection of music.
I tend to start my day with a smile on my face and a spring in my step and it is directly attributed to the awesomeness that is my morning bus ride.
On the odd occasion I take an earlier bus, the driver is usually quiet, mostly polite and will only greet me if I greet him first. My mood for the morning is definitely not as bouncy as usual.
So, when those mornings happen, I try to be the one with the smile, the happy greetings, the effervescent bounce and hopefully my mood will rub off on everyone around me.

I guess what I’m trying to say is, smile dammit!