Self help: forget positive thinking, try positive action by Richard Wiseman in The Observer,
"More than a century ago William James (brother of the novelist Henry James) proposed a radically different approach to change. Decades of research has shown that his theory applies to almost every aspect of everyday life, and can be used to help people feel happier, avoid anxiety and worry, fall in love and live happily ever after, stay slim, increase their willpower and confidence, and even slow the effects of ageing (my italics)."
It's all about going beyond positive thinking and using positive actions to influence your mood and your health, and even other people. So, don't just think positive thoughts to feel happier. That might not be enough. Force a smile and it will make you will happier.
It starts out with pretending to be happy. You don't feel like smiling, but put on a fake one and then it turns real. I do that all the time! I start to daydream and worry, but I pull myself out of it by smiling. It helps to think a positive thought at the same time or turn my attention to something nice while I'm smiling, but it will work just to smile. A positive thing will come: a happy thought will pop into my head or something pretty will catch my attention once I'm smiling. It really works.
Influencing other people by nodding when you want them to agree with you sounds like it might work too. I'll have to try it.
We've all heard that you're only as old as you think you are, but there's a study where they got a bunch of men to act younger too, and after a week of this youthful behaviour, they improved their "dexterity, speed of movement, memory, blood pressure, eyesight and hearing."
Here's what the article says about smiling and nodding to encourage agreement, but there are several more examples that you'll want to read :
"HAPPINESS: Smile
This is the granddaddy of them all. As Laird's study demonstrated, smile and you will feel happier. To get the most out of this exercise, make the smile as wide as possible, extend your eyebrow muscles slightly upward, and hold the resulting expression for about 20 seconds."
"PERSUASION: Nod
If people nod while they listen to a discussion they are more likely to agree with the points being made. When you want to encourage someone to agree with you, subtly nod your head as you chat with them. Research led by Gary Wells of Iowa State University shows that they will reciprocate the movement and find themselves strangely attracted to your way of thinking."