Some of the ways to gain weight are obvious. Like eating a whole chocolate cake or sitting on the couch eating chips for a year. Sure.
But there's something insidious that I see with a lot of clients. And that's eating a *little (or sometimes a lot) too much. Eating beyond when you're hungry. Eating (only) cause it tastes good. And let me tell you, even if you're working out, even if you're eating healthy food... doing that really erodes your progress.
I'm not just talking about portion size. I mean eating when you're not physiologically hungry anymore. I saw an interesting analogy. It's like continuing to pour gas in the car when the tank's already full.
Start to notice your physiological hunger. Stomach lightness, growling, things like that. Eat only when you're hungry and stop when you're physiologically full. Like medium-belly fullness. You can still massively enjoy your food. (Please do!) If you're eating something delicious, you can absolutely wrap it up and eat more when you're hungry again.
But listening to these signals (and getting to know them again in some cases) will make a huge difference in your weight loss efforts.