I heartily agree. Getting someone to do you a favor, even a small one, is indeed a very effective way to gain trust & build rapport. When I first learned of this concept I was skeptical, so I made a conscious effort to see if it would actually work at my job. Indeed, even a simple "could you grab me a cup of coffee while you're getting one for yourself" favor resulted (usually) in a slight uptick in the other person's perception and treatment of me.

The human psyche is pretty fascinating at times.