When did you last help someone else?
How often do you go out of your way to say thank you?
What is the kindest thing you have done this week?
Helping others, expressing gratitude and practising kindness regularly, but in a felt and meaningful way, all increase our own (and others') happiness.
See http://health.msn.com/health-topics/depression/do-good-feel-good?page=1 for more on this subject.