Edit (June 17th, 2013): here's a link to what I sound like / me being a dork: https://soundcloud.com/sound-sybil/voicesample
There's also an example of how I sounded 5 months ago there to reinforce that it's an ongoing process for me.A short introduction of my circumstance and what my voice has been exposed to:
I am 27 years old. I started to work on my voice when I was 25, but quickly became discouraged and took a long hiatus, where I picked it up again from scratch when I was 26. I use my voice quite a bit among strangers online, which for a while was my measure of passable or not. To risk a lack of modesty, I am very frequently told that I sound cute or attractive and am extremely pleasant to speak with, which has given me a great deal of confidence and encouraged me to share my developments; it has all been very rewarding, and I really hope that others can experience the same.
What my learning process looked like:
A friend: I had a friend that I spent a great deal of time on Skype with, and he didn't mind me experimenting with my voice. He also gave me a lot of feedback when asked. It helped to have something to do with him, in order to keep time rolling and encourage my subconscious to keep up with my voice.
Muscle groups: I spent a lot of time focusing on the muscle groups in my throat. Top, bottom, front, back. I did these individually. Focusing on top and back (i.e. underutilizing front/bottom) seems to have the best effect, but learning to do it gently was important to not sounding strained.
Vocality: My voice originates primarily from just behind my vocal cords and attempts to resonate over them. This is difficult to explain; the best way I can describe this is maybe to say it's where the throat tightens up most at the apex of a hiccup. Tightening this area further is how I yell or scream.
Intuition: I tried to follow lots of online advice and instruction, such as speaking in falsetto or repeating certain phrases. I found this to be incredibly distracting and it over-complicated the process for me. Eventually I abandoned these methods and simply tried to sound as genuinely female as I could via intuition.
Coughing and laughter: I cough from the same hiccup area mentioned above, making an effort to aim it towards the front of my mouth. Laughter originates from that point as well, but I make sure to send a bit of it into the top-front of my throat. This makes it sound more authentic and helps to mute a sort of chipmunk effect.
Miscellaneous:
* adding slight breath to my voice softens out any nasally sounds, but I use only a very slight amount -- otherwise I sound like a phone sex operator, which has obvious drawbacks in
most situations.
* speaking towards the front of my mouth makes me sound more girlish (as opposed to womanly). I think this is primarily because it narrows my vocal cords. Regardless, I've found it useful to achieve that sing-song quality that most women are guilty of.
* avoiding an echo/resonance in my cheeks makes my voice sound thinner. This helps me to sound younger.
* freezing (not utilizing) the front of my throat can be incredibly helpful, especially during sharp or tough sounds: laughter, yelling, higher pitches.
* this seems obvious, but not smoking. I started a few months ago. Now, my voice is often dry for the first half of the day and I encounter an excessive amount of mucus some times, both of which tend to make my voice break or rasp. I'm sure a few years will damage it beyond repair, too. I really need to quit. A lightly related note here would be to stay hydrated.
I think everyone's voice is really different. Though I still look for ways to refine it even now, I managed to wrangle a female voice in about a month's time. I don't know if this is because I learned exceptionally well or simply because I was biologically predisposed to have an easier time. I mention this because I think it's important to recognize that any advice received is not a magic or even correct answer. Not everything worked for me, but something did. I think intuition and the acceptance of individuality are extremely important to success in this subject.
I do some times worry about the health of my voice. During my time learning, I put a lot of strain on the various parts of my throat. So far as I can tell, my voice is not damaged from my experience, but I wouldn't say it was completely safe. I think it's important to be gentle, but I also think it's important to understand that the body needs to acclimate to doing something it doesn't normally do. Like with any training, that means prodding the body beyond its current limits.
I realize this was quite a bit to read over, but it was a very involved journey for me, and I would very much like to share for/try to help anyone still searching for their voice. I truly hope that anyone still looking discovers what they're searching for; the absolute best of luck to them.