I'm slim and I'd say the thing that helps is a lot of protein which fills you up and is good for energy and strength. I also only eat full fat dairy, and mix fruit veg into my cooking rather than stand alone as I find salads boring and need heavy dressings or sauces to liven up. Tbh I do wonder if a lot of it is also genetic as I think I have a fast metabolism like my parents so burn calories very quickly.
Breakfast is porridge or muesli with honey, bananas, raisins, nuts or whatever fruit is in season. A lot of milk with it. For fry ups I choose bacon over sausages as I think they're lower fat and grill where I can. Lunch when I'm home is an omelette (I eat a lot of eggs) with whatever spare veg/meat and cheese, at work I do meal deals but will pick a sandwich or wrap with chicken and veg in it, and little/no mayo. Dinner is one pot or bake recipes like curries, pasta, casseroles with salmon or pork/beef - oily fish and red meat are important for omega 3 and iron and fill me up. It's easy to add veg to these dishes so it's all one meal. I'm Asian so rice at dinner is standard or pasta but I choose whole grain/whole meal. I love carbs and think portion control is more important than wholesale cutting things out. I don't like creamy sauces so rarely add it to food (maybe sour cream) but prefer tomato or coconut milk bases. I only cook with olive oil unless it's a special meal. Also don't like deep fried anything, prefer grilled or pan fried.
I don't deny myself puddings and wine but portion control it - so a few squares rather than whole bar, 1-2 glasses with dinner rather than whole bottle. Prefer deserts like mousse to heavy cakes (rather get my calories from chocolate and dairy rather than sponge), and I will choose a fruity desert if possible. I don't tend to snack during the day but when I do i don't worry about it being healthy tbh as it's my treat and my other meals are healthy.
The biggest weight manager for me is staying active as part of my daily routine. I don't like the gym so do a LOT of walking, always take stairs, gardening, carrying heavy groceries home etc. I do run, bike and hike but it's only at weekends or holidays. I do body weight and dumbbell exercises at home to maintain physical strength - the Sweat app is great for this.
I am partial to take aways and do eat out often but try to go for something with protein and vegetables so prefer Chinese or Thai to a curry or pizza.