Whew, not going to be a popular answer. Or it may be the most common, not sure.
I lie (sort of, to others' perception it may seem so).
My reality is, I have always been a girl. End of story. I avoid anything indicative of the opposite. I refer to myself as female even when describing my past. I went to girl scout camp and in general led a normal young woman's life.
This sort of came into being without intent. I just found myself fudging details that did not fit "me" until the new fabricated truth became as solid as the original truth to me. This is not the kind of thing you can change up, you have to have the solid, non changing basics down.
As Jennifer Echols said, "It's easier to remember your lies if they're close to the truth," and so it is; in reality, at least in my experience. Thus all my "lies" are based around twisted/changed/altered truths. An exampled would be the aforementioned camp. When I was 12 I actually went to a Boy and Girl Scout camp, boys on one side of the lake, girls the other. It is very easy for me to relate things I saw girls doing or shared in experiencing with them; as I experienced them myself as well. I just fib about which dorm I slept in and who I told fireside tales with. Similarly do I handle secondary schooling, work, dating, prom etc. Derivations from the truth but so close remembering them is never an issue.
Also according to "channels", such as Darryl Anka who channels Bashar, by accepting a different past as your own you change reality streams to make that your reality. Though in all literalness I am aware there is not a quantuum shift nor any significant change in my world, I can relate to the base concept this self help personae is getting at.
This is actually basic human behaviour, an interesting factoid I learned from my critical thinking English 101 class was that people alter their memory of past events to fit their world view and that doing so is actually a part of how we maintain a healthy psyche by allowing us to explain/fit/accept things that have occurred and how they have affected us. And that this altered perception of events is to us, "what really happened" even if altered by our minds. So; for me, it is the truth. Probably at this point truth enough to Poly. We are talking ingrained responses, no delay in response times and perfect consistency. Because to me, it is the truth and not a lie, to my mother however it may seem a fib in some respects.
But yes, slight fibs in the end to the eyes of some.
Kami