I poked around the coming out section here on Susan's and around the net and put together a printed package for my employer, such as a general overview of transsexualism, what transition entails, and what I would expect by coming out (pronouns, etc; bathrooms aren't an issue at my work), plus a personal letter - basically the same letter I wrote my family but took out the more personal parts and specifics about hormones and surgery -- figured work didn't need that much detail, LOL.
And I gave them a time frame. The manager I gave the packet to was supportive, so that helped, and I told her that physical changes were already starting and I wanted to come out as they were going to be much more obvious in the coming months.
Jay