The New York Times has an interesting short article on psychology studies that have looked at the emotional influence of brief touches.
The evidence that such messages can lead to clear, almost immediate changes in how people think and behave is accumulating fast. Students who received a supportive touch on the back or arm from a teacher were nearly twice as likely to volunteer in class as those who did not, studies have found. A sympathetic touch from a doctor leaves people with the impression that the visit lasted twice as long, compared with estimates from people who were untouched. Research by Tiffany Field of the Touch Research Institute in Miami has found that a massage from a loved one can not only ease pain but also soothe depression and strengthen a relationship.
In a series of experiments led by Matthew Hertenstein, a psychologist at DePauw University in Indiana, volunteers tried to communicate a list of emotions by touching a blindfolded stranger. The participants were able to communicate eight distinct emotions, from gratitude to disgust to love, some with about 70 percent accuracy.
“We used to think that touch only served to intensify communicated emotions,” Dr. Hertenstein said. Now it turns out to be “a much more differentiated signaling system than we had imagined.”
The piece goes to to discuss research on sports team success and touch but it misses out one of my favourites studies in the area, that we covered back in 2007.
It found that a light touch on the arm increased the chances that a male research assistant would persuade a woman in a nightclub to dance with him, or to swap phone numbers.
UPDATE: Christian just sent me this study finding that a light touch from a doctor encourages patients to take medication properly:
Although the positive effect of touch on compliance has been widely found in the literature, a new evaluation has been carried out in a health setting. Four general practitioners were instructed to slightly touch (or not) their adult patients who suffered from a pharyngitis when they asked them for a verbal promise to take the prescribed antibiotic medication. One week later, patients were solicited at home to evaluate the number of tablets that were taken. Greater medication compliance was found in the touch condition.
Link to NYT piece on the psychology of touch.
Mind Hacks 
Perhaps the social inhibitions on touching (superiors/inferiors etc) had something to do with not gaining influence, then………
I appear to have mirror touch synesthesia, an annoying ability to feel it when others touch themselves. One side effect is that weeks or months later, I can remember when someone physically touched me and, if I’m fond of the person, I remember when that person physically touched themselves. One example is that a close friend of mine touched his face 4 times between his lip and nose; I felt compelled to touch *my* face 4 times, which I was certain he’d notice. When I brought it up a couple of weeks later, he had no memory of the touching–his nor mine. I, on the other hand, could still remember the feeling. If someone touches me sympathetically, I remember it. It would be interesting to research we mirror touch synesthetes and whether touching makes such a profound difference in our behavior.