I think the main thing is to eat regularly, healthily and watching portion sizes. I am now 36 weeks pregnant and 67 kgs for 1.68 mt, but normally I would be 54 kgs and, bar pregnancies, I have been that weight since 2002.
My sister and my mother have also been the same weight since adolescence and my mother is a doctor who specialises in eating disorders. Here's what we do.
Always eat at least 3 meals, never ever skip, especially not breakfast. Each meal is based mostly around a starch staple or pulses, with lots of vegetables. Meat around 2/3 times a week, fish 3/4 times, cheese usually with salad or as a condiment rather than as a main part of the meal. Always eat fresh fruit at the end of a meal and therefore plan what we eat to leave enough space for at least one piece of fresh fruit, usually two. My mother and I also tend to have a little treat at the end of lunch and dinner most days: one or two pieces of dried fruit or dark chocolate, or maybe an amaretto.
Portion sizes: I come from outside the UK and find UK portion sizes way bigger than in my country. For example, when we cook pasta (which is most days) I do 80 gr for a woman and 100 gr for a man. Also, we use much less sauce than is customary in the UK. And the sauces are always homemade, which usually means fresher, tastier and less fatty than bought ones.
Always eat at the table, talking to others or paying attention to what we are eating. No reading or watching tv, otherwise one does not enjoy the food as much and ends up eating more of eat.
Enjoying the food is key: it must be lovely or I'd rather not eat it. So we probably spend longer shopping and cooking than the average Brit. For me the key things are fresh crusty bread baked that day (am a huge bread lover), ripe fruit and vegetables that are in season (so that they ripened while attached to the plant, not in a crate) and fresh or frozen herbs (dried ones have less flavour, so the dishes end up needing something else, usually fat, to become tastier).
No snacks between meals, no soft drinks other than water or juice that has just been squeezed (by us in the kitchen). Only good wine- and again if it is not nice I'd rather stick to water.
Here's an example of what we had yesterday:
Breakfast: tea, two slices of fresh bread with orange marmalade. DH makes the bread, so the slices are quite big, prob like 4 of bought bread. On the other hand, his recipe uses a spoon of oil for about a kg of bread, so v low fat.
Lunch: homemade pizza with mozzarella, tomato, basil, oregano and black olives. I had a slice that was about one and a half times my hand, followed by two apples and two squares of black chocolate with hazelnuts.
Dinner: spinach and ricotta ravioli (250gr pack shared between DH, me and DS,2) with butter and Parmesan, followed by salad, grilled zucchini, some leftover boiled potato and a slice of cooked ham. Banana.
The plan for today is same as yesterday for breakfast; pasta with tomato sauce followed by veal escalope and salad for lunch and probably mushroom soup followed by spinach and ricotta pie for dinner. Pudding to include yoghurt at either lunch or dinner and fruit in each case.
Hope this helps. I think it is really important to enjoy what you are eating and to be aware of portions as well as of what helps you stay full and satisfied until the next meal. And, as others said, be aware that it takes 15-20 minutes for the fullness message to arrive to the brain, which is why it is important to take your time while eating and to aim to stop before you feel completely full.