I've recently noticed a lot of people who are very concerned about their voice on this forum.  Training your voice can seem hopeless and impossible, but anyone who has trained their voice long and hard enough will tell you that it's completely possible to find your female voice.  I found an audio file of me singing/playing guitar (house of the rising son) 4 years ago, and I made an audio comparison of what my voice sounds like today if you match it against my singing 4 years ago. 
Honestly, I anticipate remaking this video... I made this a week ago and didn't post it because I wasn't happy with it (lack of detail in the video), however I've had horrible allergies for the last week and can barely speak period.   I noticed so many people having issues that I decided to post it anyway for the time being.  So, there is a good chance I'll remake this video when I'm not post workout in my sports bra or pretending to sing along.. but for now I thought I should post to show people just what is possible with voice training.  I taught myself to sound like this by using my piano and creating some techniques of my own.

http://tinypic.com/r/w8nuox/7  is the video link

I've helped quite a few people with their voice, and tips I always give them are:
1.  If you have a sister, imagine the way she speaks, how she moves her mouth, her hands, her face and so forth.  Chances are, you have similar mouth movements/mannerisms as her, and her voice should be very easy to imagine in your head.  Use her image for guidance.
2. Take a piano, hit 1 note over and over and practice speaking completely monotone.  Then go up 1 note and do the same thing, and continue until you can't go up any higher.  A huge problem I notice with trans girls is inconsistency.  They have certain words that sound amazingly female, but then they immediately drop back to a word they make in a male sounding voice and it destroys the feminine presence.  So, practicing mono tone will teach you to not fall below a certain point. Ideally, you want to practice a sentence where each word you say goes up a note... and then go up 2 notes in the same word =)
3.  Eat a halls.  Gargle some mouthwash.  Eat taco bell... anything you can to open up your throat and not strain it.
4.  Go download Skype and find some random online game, meet boys and tell them you want to skype chat.. practice on them.  It will build your confidence and get you use to maintaining your voice for hours.  If they figure out, who cares... block them and meet another boy =)
5.  Record yourself and critique yourself.  Do not feel that your voice is to the point where "oh I think it passes now" and then stop.  Keep working on it, why settle for good when you can have great =)
6.  Imagine the tops of your vocal chords pinching when you speak.  It will sound funny at first, but will really get you use to pinching those muscles.  That's the area that women speak from, and the area that you will have to learn to speak from. 
7.  Your mouth is a horn, and your facial expressions essentially determine the notes of that horn.  The way you move your mouth, your lips, your tongue, eyes, cheeks, jaw and so forth is going to determine how your voice sounds... so move them as a female.  Again, imagine your sister or your mom and go from there.  Female facial expressions are the key to a female voice, and to a voice that matches you =)
8.  Smile when you talk, show your teeth and be happy... being happy makes others happy, and it's easier to talk in a higher pitch if you are happy.  Imagine yourself about to laugh and go from there.  Smile conveys confidence, and confident speakers are passable speakers =)


I'll be more than happy to help anyone that needs it... after my allergies go away of course =)