I like Joanna's too.
In those cases where I must reveal it (medical professionals, pre-dating) I say that I'm a transwoman and I always add that I'm a woman who has a different social and biological history.
I would add that I would never do this on a date but always before.
As for coworkers, I decided a coming out letter at my university would be best . Here's a copy of the email I sent at the end of the spring semester of 2013 before coming back for the fall.
Dear friends,
I'm writing because I wanted to let all of you know that over past several months I have come to terms with being transgender. I have been struggling with this issue for as long as I've had memories. Like other transgender people, I experience deep and chronic discomfort and unhappiness with having to live in my socially-appointed gender.
With my family's support and under the guidance of professionals in the field, I am transitioning to an authentic version of myself, and this fall I will be returning to work in my more natural state as a woman. By that time, I will be going by the name of Jane, although you may call me that sooner if you would like.
Despite changes to my appearance, I will remain the same dedicated colleague, teacher, and scholar, committed above all to excellence in educating our students. I like to think that this discovery has made me more aware of and responsive to the countless, often-invisible struggles our students, like all humans, endure.
It is perfectly natural if you have questions for me, and I hope you won't hesitate when it comes to asking them, especially if they might help us in our constant effort to meet our mission of developing a "[here omitted to protect anonymity]." I would add that with a change of this magnitude some initial awkwardness is completely normal. In fact, members of my family were surprised and at times have struggled with the change. I fully realize mine may be a difficult decision for you to understand, and I will try my best to assist you where I can.
Since coming to _____________, I have been struck by the real sense of community that we create daily in our acts of kindness toward one another. From those of you who have already learned of my changes, I've received a quality of support that makes the terms "complete" and "unwavering" seem like understatements. I cannot thank you enough. I'm really proud to be part of this thoughtful and caring community, and I hope to know so many more of you better than I do.