There's a simple trick that makes people more likely to say yes when you ask for a favor: Tell them why you need their help.

Back in 1977, a researcher at Harvard had people go to a library, and ask if they could cut in line at the copy machine. However, they asked in a couple different ways. 

First they just said, "May I use the Xerox machine?"  Sixty percent of people let them cut in line.

However, when they asked: "May I use the Xerox machine, because I'm in a rush?" Ninety-four percent of people let them cut in line. 

In other words, people are 34 percent are more likely to do you a favor if you give them a reason

The reason you give doesn't even have to be legitamite. They also tried asking, "May I use the Xerox machine, because I have to make copies?"  Which didn't really make sense because obviously they needed to make copies.

Although when they phrased it that way, 93 percent of the people let them cut in line.