Here's a good example:

https://www.eatingwell.com/article/7922773/best-way-to-lose-weight-and-keep-it-off-according-to-experts/

30% of the people studied maintained the weight loss for at least a year or more.

What was the single most important factor?

Exercise.

Yes, exercise does NOT do much when trying to LOSE weight. But in the maintenance phase, it makes it MUCH MUCH easier to maintain the weight loss.