Asking for a Favor: The Three Keys - Jodi Glickman - Harvard Business Review
Michael Simmons stashed this in How To Be A Super Connector
Asking for favors can be hard. This article gives tips.
Tip #2 resonated the most for me.
Give a Reason
In Robert Cialdini's iconic book Influence, he showed that if you ask someone to do you a favor, you have better luck when providing a reason; people react positively to the word "because." Even if the reason makes no sense or is unrelated to the request, people like to know why they're being asked to do something. Presumably, your reason does make sense and is directly related to your request — so go ahead and share it.
One can imagine having even less success with the request above by simply stating "Anthony, can you cover for me at the client dinner tonight?" Your odds of Anthony complying just because he's a swell guy are probably pretty low.