Here is a summary of some of the advice he gives on increasing your happiness, based on scientific research:
(1) Spending money on material things doesn't make you happy, but spending money on experiences, such as holidays, shows, or even a meal, does. The key things seem to be that experiences offer the opportunity to interact with others, or having things to discuss with others after the experience. Equally, spending money on others does more to increase happiness than spending it on yourself.
(2) Smiling, sitting upright and generally ACTING happy, can make you feel more happy. The smile can't be fleeting, it must be for between at least 15 to 30 seconds or more.
(3) 'positive thinking', psychotherapy, or even just talking about your problems does not seem to make people happier. Instead, evidence suggests that writing a diary about your problems can be extremely beneficial to your happiness (the theory is that writing encourages one to structure your thoughts whilst chatting about them, less so).
(4) Similarly to (3), even if you don't have significant problems, keeping a diary in which you express gratitude towards all the good things in your life, positive thoughts about the future (e.g. imagine - realistically - things have gone as well as you can hope, and you have achieved your goals), and affection towards those you love/have loved. Part of the reason for why this works is that the human brain has a tendency to quickly 'habituate' to almost anything and thus we take things for granted and must remind ourselves of them.