I've worked on and with various Domain Specific Languages (DSLs), but I've found designing them to be the same process no matter what type of DSL is being created.
In March of 2005 I started one of the more technically interesting projects of my life. ThoughtWorks had been hired to design a system that would allow the director of finance for the client bank to edit, test, and deploy the business rules of the system without any assistance from IT. It was a bold step, but the return on investment was realized within weeks, not months (or worse, years).
One of the major challenges of designing this type of system is determining what the language should look like. Being new to the idea, we started out by telling the business what was possible. That was a mistake. The early versions of the language were easy for IT to work with, but were clunky for the business. The result was clearly suboptimal. As we continued evolving the language for new features we took a different approach, we asked the director what he wanted the language to look like. The more he shaped the language the easier it was for him to take ownership of the domain logic.
We figured out that we didn't need to write the domain logic in our language, we needed to make the domain language executable. The resulting project was a huge success, which led to me coming up with the term Business Natural Languages and presenting the idea at several conferences*.
Almost 3 years later, I decided I needed a new framework for unit testing. The primary motivation for designing a new framework was my desire for a more syntactically elegant way of defining tests. This time I knew I wanted to design what the language looked like and then worry about making it work at a later time. In fact, when writing expectations, I had a small suite of example tests before I had any way to execute them.
None of the languages I've helped design are flawless. But, I've seen significantly more success with the languages that were designed by a subject matter expert concerned with the language, instead of a language designer who was concerned with how to make the language execute.
* If you'd like to catch the Business Natural Language presentation you can see it at QCon London 2008 or rupy.eu 2008.