One of the best ways to maintain I've experienced is to just keep track of your weight every single day. And trying to do it while knowing daily fluctuations don't matter much, but that if over time you've gained a few kgs, you need to figure out what happened wrong and how to fix it.

I had a daily habit metric to "eat well" but that was much more complex and mentally taxing, so I simplified it to "track your weight" which is just a minute of concrete simple investment every morning. Of course, weight isn't the only thing that matters, but if mostly you're eating healthy and quantity is the only issue then it's a useful metric to track.