Always smile and share it with others. If you see someone you know "smile" and they will return it back to you. But here's the thing, you may not notice but this smile you share may affect them and lighten them up a bit and received some positivity.
 Because by simply smilling you are showing and expressing many things.  when you smiled at some one, it means you acknowledged them, your remembered them, you noticed them, ( you will never smile at someone your mad at, right hehe) by smilling at some one it means your are acknowledging your friendship with them and you are showing to them you are some one they can talk to and lean on. 
So basically, smilling is not just smilling. It is a very a short and a very good way to share good vibes and positivity to others without doing much effort. So keep smilling and spreading positivity every day.