If this was your last day, would you be happy with what you accomplished, what you contributed to the world?
It's the little things. Did you listen to your loved ones? Did you take the time to really listen and hug your children? Did you contribute to your work with as much passion as you wanted to? Did you say good night with nothing left unsaid and no one left upset? Were you content with your actions and your words towards other people?
Did you sing or dance? Singing, whether you are a vocalist or not, is an expression of your soul. You must sing - in the car when you are by yourself or out loud with a huge audience. You feel better. I am a scientist, but my whole life would be very empty without the expression of the spirit. If you haven't carried a tune in a long time, try it. Hum while you walk or email.
Sing, work, play, laugh, love, stay good... everyday...like it's your last.
"Live life with your head, each day with your heart."
That is happiness.