We hired an external design agency to help with the design of the large Lights Silverlight application. I would suggest starting the process with paper and pencil, collecting ideas, looking and feeling, etc. Also think about who your target audience is, we created Personas that represent specific types of users, if your users have a problem with the final user interface / UX is bad.
Then we let the designer create parts of the user interface in combination with nothing (or maybe just a little to test the idea or prove the point).
At the same time, you can consider / develop the overall flow of the application (screens / views / subviews) and run your infrastructure (that is, which MVVM suits you, how to cut out views using controls or usercontrols, etc.).
Bringing visual images along with code that we then ran at intervals. It turned out to be an iterative process, but with different perspectives as the project progressed, it helped smooth out the problems they presented.
Definitely talk through the process before starting.
A bit of a tramp, but I hope this helps.
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