First, you have to start by acknowledging feelings of other side, e.g. “oh…” or “I see…” Then name it simply as “that sounds frustrating!” or give a fantasy by stating something like “I wish I could make it for you right now!” By now, they should be on your side. The next step is to engage a cooperation. Try it by giving information (not an order) and/or describing what you feel. Never punish, rather state your expectation, or again, express your feeling. The feeling is always good. At last, encourage autonomy with choices, turning questions into “what do you think?” or encourage to use outside sources. As final touch you can raise self-esteem as simply as saying “fantastic”!
Let's try it with an example. Let's say some computer programmer is saying it's not possible to do a feature you want. Say this:
(acknowledge the feeling) I see… (name the feeling) that sounds frustrating! (describe your feeling) I don't like using this software without this feature. (engage cooperation, give an information) This is very important for all users. (don't punish resistance, state your expectation) I expect to have it done after collecting money from all our users. (encourage to use outside sources) Maybe the community have a suggestion. … (self-esteem final touch) It's a pleasure to work with you!
Of course, it's not from my head. It's from the book. About kids. That book is called How to Talk So Kids Will Listen & Listen So Kids Will Talk, little bit longer name. I really recommend this book even if you don't have or plan kids yet. You know, in the end, adults are just big kids. :-)
…ok, I used little bit of Devil's advocate here. I don't agree this is manipulative technique. It's just the way how to be more nice on each other. At least I think so. ;-)