When are moments to include the client in the UX process and when is it better not to? I am talking about after kickoff. Would you include them in analysis of card sorting, tree sorting? How about defining content? What is the most important advice you can give about including the client in the UX process?
If you are talking about client as the guy who pays you to do usability evaluation, I think you should NOT include her in your jop specific tasks like, card sorting, usability testing, eye tracking, etc. I think you should rely on your knowledge and stick to the specific evaluation methodologies because they are well researched and will get you the most valuable results. The client may want to tell you how to do the usability tests, but you should stick to the best practices. This is your job.
You can use the client to tell you end-user patterns or how users behave in certain situations, and get domain specific information. There is no one that can give you better insight about the specific business than the client. It is absolutely necessary to have good knowledge about the domain you are working on. Also, it is necessary to discuss the content strategy with the client, as most of the times she will update the content.
I like to work with clients closely during the story mapping and design studio sessions. In particular I have found story mapping sessions highly informative to get the client involved, as they share many insights into the stories being sorted and created. It also helps the client to have a good understanding on the scope of the project and how much work is intended to be done overall. Including clients in design studio sessions helps alot - as they can contribute to the ideation process, where they can share some of the concepts they have in mind for the solution(s).