Baruch Sachs makes some good points in Part 1 of a series called Practicing Great UX Consulting:

You are not there to educate people. You are there to advise your client and guide the creation of an amazing user experience. You are the expert; that’s why they brought you in. Collaboration and openness are key here. People need to feel invested, not put upon.

I agree that collaboration is key — the problem comes when collaboration gets confused with consensus. Consensus cultures often produce watered down, unexciting products. Products where endless rounds of give-and-take have worn down the original idea to a shadow of what it once was. Consensus cultures also wear down the teams working on the product, because no one really gets what they want, they just get some of it.

Collaboration is different. In collaboration cultures people understand that even though everyone gets a voice, not everyone gets to decide. People are able to voice their opinions, argue passionately for how they believe things should be done, and try to negotiate compromises. But it certainly doesn’t mean that everyone has to agree with every decision. And that’s why the client/agency trust relationship is so crucial.

One great collaboration technique that works well with clients is called Design Studio. Jared Spool has a great write-up of how to conduct a Design Studio worskhop, and it’s also worth reading Paul Boag’s thoughts in Never wireframe alone.