While I do not know the secret to eternal happiness I can tell you some tips for short-lived ones. In a life that is anyway limited by the amount of CO you inhale, limited by some silly thing called age what is the point of having a secret to eternal life?
The secret of happiness is small things of joy. Now this can take many shapes. A Hi or a hello from that lovely thing next door could perhaps make you happy till you reach the office. I say that because at the office you have email waiting from your boss who is busy seeing his l.t.n.d.
One of my short-lived happiness is the green light at the traffic as you approach it. Waiting is really bothersome. For some of my friends with a poetic bent of mind a rainbow would be a source of happiness. At the supermarket my favourite make-me-happy is the one to take the last piece of free newspaper.
Take your pick, a sleeping baby, a koel singing, pitter-patter of rain, catching the train just in time. These things are soon forgotten but the feeling they give some is nothing to be pooh-poohed. The word that comes to my mind is ephemeral.
Those moment that makes say Yesss. I leave you with some images.