@Alex - why don't you share your narrative? It'd be good to hear various stories.
For me...it's been a process, really. I mean, throughout my life there have been various things that I now see as obvious signs of me being trans, but I just didn't connect the dots. I have a memory from when I was REALLY young (think: 3-4 years old or so) that I felt something was missing in my pants, so I'd stuff my underwear with toilet paper. I just didn't know WHY I did that. Or why around age 10 my mom made me put on a shirt when guests were coming over (I loved that I could just run around shirtless at all times before then). Or why I was most aroused by straight porn later on even though I was most often attracted to girls. Or why the most exciting thing for me about fitting the "lesbian stereotype" was appearing male. Or why when auditioning for school plays in high school I always chose to read the male parts. Or why I liked one of my friend's nicknames for me - "little boy" (started because I liked the Smashing Pumpkins' song "Disarm," which has the lyric "I used to be a little boy" combined with the fact that sometimes strangers read me as a preteen boy). Or why I was jumping up and down, laughing, saying "this is me" when I dressed up one time to record a silly show thing with my cousins in which I looked totally male. I also remember thinking at about 18 that I REALLY wanted to get a hysto to get rid of my period - what stopped me was fearing that if I later decided I want to have kids (like I plan to), I'd regret the hysto.
And that whole sex thing REALLY confused me so much until recently... I kept thinking things like - what the hell, I'm a girl, almost always attracted to girls, but I keep fantasizing about heterosexual sex. What I didn't really get was that I was turned on by the idea of being a guy having sex with a girl, not the other way around. The only reason I actually realised this is because once my girlfriend found out I was trans, she started treating me and my junk more like a guy's and that REALLY was a turn-on. Before then, even though at that point I already knew I wanted other aspects of being male, having a penis wasn't of huge interest to me. I would have always said "I don't care what I do/don't have down there". I've truly come to see otherwise in the past few months.

The first time I really told myself I might be trans was about 2 years ago now. Before that, I never really fully consciously considered it. I mean, there were all the things I mentioned going on and more, but I never thought of the fact that I might be trans. It just seemed like something far-away, impossible, abnormal - almost like a sickness (I'd never judge anyone or say anything negative about someone being trans as it had nothing to do with me, as I thought at the time, but I just had this "icky" feeling about it). I did out of curiosity a few times before that research what being FTM meant and stuff like that (heck, before I researched it, the word trans meant an image of an over-the-top drag queen in my mind). However, it wasn't until I started watching transguys' videos on youtube about 2 years ago that I started identifying with them (I guess seeing real guys talk about it changed the 'icky' perception I had of trans folk).
Since then, I've been binding on and off, but didn't start packing until the recent revelation thanks to my girlfriend (which was only a few months back), and I've only been packing with socks since that's all I could afford. Was just about to attempt to make my own packer 10 days ago (not sure how successful I would have been), when that very day I got an e-mail that I'd won a contest on TQ Nation - a packer and TranZwear boxer-briefs. The little guy should be sent on its way to me sometime this week.
So, in the past 2 years, prior to my girlfriend finding out a few months ago that I'm trans, I kind of thought this was something I would just have to keep to myself for the rest of my life and kind of "push it away" because I saw that so many guys lost their significant others after coming out. Another HUGE concern of mine is my family. Now that my partner supports me 100% it makes it easier to deal and it also makes me think I might not have to live like this and could maybe transition. I just don't know WHAT I'd tell my family. My partner suggested I could say I developed some sort of hormone imbalance where my body produces testosterone and that my doctor said it's easier to become male than going back to female through HRT (in that the changes of estrogen would be partial and I wouldn't lose all those male characteristics testosterone caused), and that I chose to stay male because of that. But yes, I highly doubt something like that is even POSSIBLE, especially in a 25 year old (i.e. why would my body SUDDENLY at 25 or later start producing more testosterone than estrogen). So, if they asked their doctor, they'd probably conclude I'm lying. So, I'm really stuck there and thus am not sure what I actually will do.
I do, however, know that I have some time to think about it, seeing how I plan to give birth in a couple of years (after finishing school). My partner does too - that way we'll both be biological parents to our children. After that, however, who knows what will happen. I might try some form of natural transition, or I might go on T, but I highly doubt I'll just do nothing. Oh, and top surgery is a MUST for me. For bottom, I'd rather get a glue-on prosthetic than any of the current surgical procedures that are available.
And, in a nutshell (a HUGE nutshell

), that's my story.