Positive thinking does have effects, but they're not necessarily positive, according to Professor Richard Wiseman.
In one study led by Lien Pham at the University of California, students were asked to spend a few moments each day visualising themselves getting a high grade in an upcoming exam. Even though the daydreaming exercise only lasted a few minutes, it caused the students to study less and obtain lower marks. In another experiment led by Gabriele Oettingen from New York University, graduates were asked to note down how often they fantasised about getting their dream job after leaving college. The students who reported that they frequently fantasised about such success received fewer job offers and ended up with significantly smaller salaries.
Wiseman recommends positive actions, like forcing yourself to 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.
A previous question here pointed to a study on values affirmation, where having physics students write about their most important values raised the exam scores of female students, but seemed to lower the scores of male students.