I tried to do a bunch of small things. No single thing was the key.

• started drinking my coffee black
• drink more water, and drink water instead of other things
• I take the stairs instead of elevator/escalator whenever possible
• I try not to eat seconds at dinner
• "Real" exercise (like running, rowing, etc.) every other day, but if I can squeeze in extra ones, (e.g., 2 days in a row), I go for it
• If I'm not up for real exercise, I'll go for a walk
• intermittent fasting in the mornings, so often no breakfast, but not necessarily every day
• No snacking after 19:00
• At restaurants, if 2 options look good, go for the one with fewer calories

There are probably more, but those are the kinds of habits we're talking about.

Also important: if you don't stick to all habits, don't beat yourself up about it.

With the above, I was able to get from a high of 235 lbs down to around 180. I'm a bit more than that right now, but pretty close still.