The key thing is to do/start with the same thing everyday, with slight variations in the end. Make it an autopilot routine during which you can be mentally elsewhere. Only then you may think “I have to…” and [click] it’s done before you even notice.
That's what the entire internet tells me, but it just doesn't happen to me.
Whatever it is that I wish to keep up, I have to decide to do it every time, roughly the same as I did the first couple of times.
Yes, that includes brushing the teeth.
Of course there are habitual things in my life, but I can see no common characteristics. Total time spend certainly isn't it.
Amusingly, this has made it almost trivial for me to quit smoking despite being a smoker for many years. I just never developed a habit for it, so dropping it was as simple as just not smoking.
But actually, surprisingly, things you don't initially want to do can become positive habits in a very short time period. Like, 3-5 times. Of course, the flip side is also true; I've often fallen OUT of workout habits just as easily. But at the end of the day, perfection is not the goal. You shouldn't avoid starting a new positive habit just because you know you'll probably lose the habit again. You can always pick it up once more!
Someone who is a regular gym-goer for a few years, then becomes sedentary, then picks up running for a half decade, then moves on to cycling, then falls out of the habit, is probably better off than someone who never bothers to try.