Smaller plates help with portion size.
DS is 2 and we walk a lot locally. I know he's much younger, but we do things like hunting for shapes as we walk which make it fun. I avoid driving short distances.
Slow cooker is great, especially for warming hearty winter food.
Don't go down the junk aisles of the supermarket. If you can't avoid, fast track through without looking. Make sure you're not hungry when you shop.
What do you drink? I struggle with artificial sweetners so avoid them (they can trick your metabolism/ appetite by messing up the signals of sweet taste/calorie intake). I drink very, very weak squash. Diluting fruit juice down is also good. As I'm trying to lose baby weight, when I'm out, where I can I swap coke for fruit tea and am drinking it more at home to cut the sugar.
Check labels. Different brands of the same product can vary significantly for things like sugar and fat content. You're better with the natural form of the product than reduced fat/ sweetened. E.g. Low fat yoghurt is more watery, less filling and contains more sugar than a standard yoghurt that will fill for longer.
We don't have much of a snack/ pudding culture in this house anyway, but it was taken out of my control by DS being allergic/ intolerant to milk, eggs and soya which means a lot of processed food is out, and eliminates most sweet and empty snacks. He's never been able to try chocolate. I often feel like a right killjoy having to decline x, y and z and check labels, but his life is no less fun through a lack of refined sugar.
If your family try to guilt trip on you, place it back by reminding them of the long term benefit of saying "no", that DD will have more fun being fit, that she'll be at lower risk of diabetes/ heart disease etc, that she can enjoy buying trendy mainstream clothes...
When we need to pay attention to our weight/ health, we tweak a few things around (like the coke- fruit tea swap) and it makes a subtle, easy but useful difference. It's sustainable because we don't feel deprived.
Sometimes I have to focus on the long term result. I gain a lot in pregnancy, and become very immobile (SPD), and I would hate for that to be my long term future. Being able to be active with my DCs and hopefully their DCs is a powerful motivator.