Thought I would answer in detail here. Indeed, I am working on this kind of thing right now, as I think this is one of the stones I have not yet unturned.
My house is full of handmade affirmation cards that I have made and tucked in places I will see frequently, like light switches. Some are based on suggestions from the fertility hypnotherapist I went to, and some from the book You Can Heal Your Life that I am reading right now, as well as the charming Power Thought Cards card deck by the same author.
However, I didn't make an affirmation card specifically targeting the fibroids, DUH! Thanks so much to the commenter for suggesting it. I just made one that says "My uterus is beautiful and smooth, perfectly shaped. It's the perfect safe place for a baby to grow." and have put it near my desk.
The commenter also mentioned the work on Creative Visualizationby Shakti Gawain. That looks fascinating and I will have to add to my reading list.
About visualizations, I will share one that I made a year ago at the time of my diagnosis. I imagined my liver as an actual janitor in my body, but as a kind of flawed character, a blues singer type who hits the bottle a bit too hard. So my initial visualizations focused on helping him get himself cleaned up, so that he could then work on getting the fibroids out of my uterus, which is what I was trying to do on the physical level with the detoxing. I haven't been checking in with him so often as my liver seems to be much better, but maybe I should be...
I think visualizations must work, based on the experience of a friend. She had breast cancer and was given 15% chance to live. She did visualizations intensively (in addition to chemo and lots of Chinese herbs). Over 10 years later she is in remission and doing great!
Finally, I have a coach that I started working with a year ago when I got my grim fibroid diagnosis, and she is very talented with leading meditations and creating visualizations. We are working on a really exciting special project related to visualization, that I will share with you very soon, stay tuned!