You don't need to be so polite. Business is business.
I'm not suggesting being rude by any means, but being forthright and upfront will serve you best. You pretty much have to provide an ultimatum. There's no way around it.
Here's how I would handle it....
For a gentle nudge......
I'd really love to get started on [project name].
Has there been any decision regarding confirmation to move forward? Are there more concerns you may still be deliberating?
Year's end often presents more work than my time will allow and I would love to ensure we're moving forward and I can dedicate my time to your project.
Essentially you start dropping hints that you're busy and they need to jump on the fact you are available. With every conversation try and insinuate that if they don't grab your time, it may not be available.
I'd love to get started! I'm concerned my window or opportunity to work on this may be drawing near.
Any progress with the signing of the contract? I need to schedule my available time and I'm unsure if I should factor in this project.
For a more definitive deadline....
This time of year often brings in a great deal of work for me. Often, much more than my time will allow. To this end, I need to pick and choose which projects I feel I can best serve.
I would truly enjoy the opportunity to work on [project name]. However, in order to sequester the time necessary to complete the project I need to confirm we are moving forward before [November XX, 2017]. Otherwise I'll need to move on to other opportunities.
I would be very happy to discuss concerns or answer any questions you may still be deliberating over. I really do want to work on [project name].
Unfortunately, I need to consider scheduling.
Thank you for your understanding.
With that... they either confirm by the date or you move on.