I’ve always been naturally slim (excluding 3 pregnancies) as have my parents.
I suppose my three habits are:
Cooking everything that I eat. Rarely eat out for financial reasons, but I’m also a really good cook. At home I can control the amount of oil/butter etc. in my food, and control the portion. I’ve always done this.
Moving all the time. I never sit down. I’m a teacher which involves moving around the classroom all day, I’m very active and walk/cycle/lift weights whenever I can. My 3 young DC keep me on my toes all day. Both my parents are the same, you will never see them sitting down. They are always gardening, DF is on his boat, they cycle, go for walks etc. We never watched tv in my house growing up, only reading a book would have been an acceptable reason to sit down.
Never letting my weight creep up too high. If I notice my weight increasing, due to overeating, travel, too much snacking etc. I act really quickly. I’ll be strict for a few weeks and it will drop back down to normal again. Even after 3 babies I only had about 3-4 kilos to lose after each pregnancy which I was able to do in a few months. My DH, on the other hand, will let himself get 10+kg overweight, then have to spend the next 6 months on a weight loss program. I never let myself reach that.