I won't see her for months from now but I know she's supportive and I want to tell her because the more people that know (and accept) the better I feel.
If you can't wait until you see your friend, I'd just go outright and say "I'm gay" over a message. If possible, it would be better to use a program like Skype so that they know you're not joking.
Yeah this is what I'd advise as well. Did that when I told two of my friends, just sent them a message over a site we all visit, and just told them outright. Granted I didn't check my messages, or Twitter for like a day after that but it all worked out.
I agree with what was mentioned above with Skype, it's more personal so might be better, but if that isn't possible I would just write a heartfelt email or message explaining things so she knows you are serious.
A friend came out to me online. I was the first person he came out to, even though we weren't really that close (only knew him for about a year), so I'm still puzzled about that till this day. He told me that he felt that he needed to tell someone to get the boulder off his chest, and decided to just blindly type it out on MSN (yes, this was a while ago) without thinking much about it. He found that the more he thought about it, the more difficult it was to do it. However, before he told me, he did steered the conversation towards a topic about the gay movement, and was asking me about my thoughts on the gay organisations at uni and their activities. I suspect he was testing the water because as I've said, we weren't really that close. Since you already know that your friend is/will be supportive, you should just go straight to the point without a second thought.
