Finding my voice was something that just sort of happened over time. I thought it was gonna be nearly impossible to do, even with the help of Deep Stealth's Finding your female voice. What did it for me was just spending a huge amount of time with other women. I worked long hours in a store where only women worked, which was great, and e very customer to come in through the door was another opportunity to "nail it".
Still, to this day (had my 10 year anniversary for hrt and ft yesterday ^_^), speaking in my female voice is something I "do". It's a performance of sorts, but I'm so used to it I don't have to think about it. I haven't tried to speak with the male voice in 10 years now so I really don't know what it is like anymore.
How to do the female voice (pitch/timbre/resonance) is hard to explain but when I speak it feels a little like I'm dominating a furry little animal (my voice box) under my thumb. It squirms and bucks, but I apply more pressure and keep it under control. I push it in the direction I want it to go.
Another way I like to look at it is like operating an electric grinder. As it digs into the wood and tries to get away from you you tighten the grip and apply pressure in the opposite direction. As you get better at it you know exactly how much pressure to apply and you can do it effortlessly by following the path of least resistance. Today I can even shout in my female voice, although I rarely do as I'm quiet a timid person.
There is one situation in which my voice can fail. That is when I haven't spoken for a while and my throat is relaxed. If I haven't spoekn for a while and the phone rings I sort of have to wait a second or two before answering so I can go through this little check-list in my head and make sure I'm in the right "spot" to speak. The first vowel or syllable exit my mouth sets the tone for the rest of the words. If the first word is good, then the rest will also be. It's kind of like aiming before pulling the trigger.
A good way to practice would be to set up a real-time delay/echo plugin and set it as a 1-2 sec slapback. You say a word and 1-2 seconds (depending on the delay time setting) you hear yourself in your headphones. No recording, rewinding, stopping or pressing play.
Spending a lot of time with other women though is the way to get the inflection and stuff right.