No one seems to have said it yet, but consistency. Waking up at the same time every day including weekends. Eventually, you just wake up early naturally. I get up at 5:30 am every day on the dot with zero alarm just because it's when I wake up. And I'm never groggy, because I've adapted to it. It's not early anymore it's just when I wake up
I think the second half to this is that it can't be a chore. You have to want to wake up. If you wake up and think about how much you hate being awake it will be all the harder
Winter is the same, although it can be noticeably more difficult to recover if your pattern gets disrupted for any reason. So for that I use a light that simulates sunrise