I don't know if this will work for you. Every night I close my eyes and picture a clock with the time I want to wake. Then I tell myself the time I want to wake up.
I'm.. weirdly similar. I tend to wake naturally about 5 minutes before my alarm every day, even if it's not a normal wake time. Doesn't apply if I didn't set an alarm, obviously.
I think for me it's an anxiety thing, and a generally good sense of time. If I have to be up at a certain time, I sleep more restlessly, and I tend to be good at estimating the time when I just barely wake up. So I wake up several times through the night, and just stay up when it's near time for the alarm.
I dont have this method but I do seem to be able to fall in to a routine in less than a two week period and then just wake up at the same time every day essentially invariably, not sure how I do it. I dont remember it being like that when I was a kid, but at least since my military service Ive been able to get up when needed (not saying its correlated to military life, just that thats what point in life I was at when I noticed this)