I live in a different part of the country, so a letter was better than a phone call, which seemed better than waiting for their annual visit in the summer (you can see my thread: I decided to go with a letter). I'm a writer, too, so it felt a lot more comfortable having the opportunity to organize my thoughts the way that I wanted things presented. It also let me include some information towards the end about the process. It ended up being 5 pages single-spaced. And yet, it leaves so much unsaid.
Mine went over well -- at least, as well as one can expect. We talked on the phone 2 days after; and, although she was crying basically the whole time, she's behind me 100%. Breaking it to my dad will be another story entirely.
I wish you luck. At the end of my letter I wrote:
I know that this was a lot to take in. I really look forward to your phone call. There's so much more to say. I really love you both, and hope that things only keep getting better.
She emailed me back within minutes (she was expecting the email) and told me not to worry. It's been extremely tough -- this is the most emotional of all of my 'coming out' events so far, but it's the one that I was most worried about. I'm glad it's "over," even though there are more tough discussions to be had.