This headline, from the Daily Telegraph website (12 August 2009), is wrong. Make it:
Thinking of something good that happened the day before boosts happiness.
In Standard English, verbs agree with subjects. Here, we have a subject in the form of a nonfinite clause, as underlined above. When clauses function as subjects, they are grammatically singular — hence the singular verb boosts.
The subheading is a little trickier: Smiling and recalling something pleasant from the previous day help to make you happier, according to a new experiment.
The plural verb help, if intentional, suggests that the writer was thinking of smiling and recalling something pleasant from the previous day as two separate activities, hence making the subject plural. My preference, however, would be to treat it as a single activity, hence Smiling ... previous day helps ....