I've always just done variations on my birth name, Charlie.
Back in high school when I first started experimenting with alternate personas, I called my female self Cheryl, (based on the story of a famous intersexual from the "nature versus nurture" infant-gender-assignment case that I saw on PBS once,) but I never really liked that name very much. To me, I couldn't separate that name from the image of a 40-year-old woman with too much makeup on for some reason. I wanted something that sounded younger and cuter.
People always teased me by calling me "Charlene," so that was out of the question from day 1.
I asked my mom what my name would have been if I was born a girl, and she says that it would have been Sarah Elizabeth [Last Name], after my two grandmothers. And she would have called me "Liz" as a nickname, because she liked the name Elizabeth. I agreed with her on that one, but never really liked the name Sarah. Probably because to me the image of "Sarah" was very "plain and tall." And again, I wanted something younger and cuter.
Next I considered "Charlotte," but for some reason it just didn't feel like me. It felt too much like a certain E.B. White book character, and yet again, I couldn't separate the name from that image, and thus couldn't picture it being used on me.
Then finally, the name "Carrie" came to me. I don't really remember how it did. I just know that it was another variation of "Charlie." And to me, it sounded cute, it sounded young, and for some reason it just clicked. So I started calling my female self "Carrie," and I liked it, so I've stuck with that name ever since.
When it finally came time to seriously consider that I'd possibly be changing my name in real life, I assumed that I was just going to go with "Carrie" legally. But then I had this odd dream one night where this middle-aged woman that I had never met before was offering me emotional support in a semi-condescending manner, and addressed me as "Caroline, dear,". At first I didn't think anything of it, but then the idea of Carrie being a nickname for Caroline grew and grew on me, because it would be kind of like the same thing that the name "Charlie" always was for me even though my legal name was "Charles." And while I still haven't completely decided on this, I think I'm pretty sure now that I'm going to go ahead with using Caroline as my official legal name, and go by Carrie. And since I also really like the name "Elizabeth," which was my maternal grandmother's name, (and my mom always tells me that I have her mother's eyes, so I think it's appropriate,) I believe that I'm legally going to go with Caroline Elizabeth [Last Name]. I REALLY like the way that name sounds. And of course, it's this name that gives me my username, Carrie Liz.