I almost always do fixed fee arrangements with clients, and generally don’t have too many problems with people changing the invention after the first draft.
One of the things I do to prevent too many changes is to summarize the invention very specifically with the inventor at the interview. I generally do not draft claims with the inventor, but come very close to it. I try to state the critical elements of the invention clearly and concisely, and make sure the inventor agrees that they are the important parts.
I have heard about some practitioners who actually draft claims with the inventor, but I find that I need some time for the invention to sink in before I write claims. I find that about a third of my time drafting an application is the claims.
In the interview with the inventor, I may make several attempts at describing the invention in quasi-claim language. I do this so that I ensure that I understand the invention, but I also get agreement from the inventor. I do not to let the meeting close without a solid agreement on the scope of the invention.
I started doing this mainly so that I understood the invention and could have the inventor tweak my interpretation as necessary. As I did it over and over, I realized that it was helping prevent unnecessary misunderstandings and getting the inventor to buy into my interpretation. When the inventor gets the application, it is drafted around my interpretation of the invention that we discussed at the interview. Nothing is new, hopefully, except any extensions or improvements that I may have considered.
The second thing I do is get the application turned around quickly. Getting it in their hands fast prevents too much time to expire and have them change their mind. The other, obvious benefit is that the clients like knowing that you are working on their projects. Getting a good draft of a patent application back to the inventors a week or two after the interview is about normal for me.
I do this for two reasons. What works for me is to draft the claims a few days after the interview, then draft the specification a day or two after that, with some changes to the claims as necessary. This time frame lets it fester in the back of my head for a couple days until I fully understand the invention.
The second reason is that clients appreciate getting attention. They want to know that their project is important. Most of the complaints about big law firms are the turn around time and responsiveness. Being a solo practitioner, I can respond quickly, which turns into satisfied clients.