I've got a friend who I've known since school. Always been slim - a size 10. At school she was always on the move, not a great one for sports but would always be walking fast or running (mainly because she was always late but...)
Now she's approaching her 60s she still gets into her older clothes, she may be pushing towards a size 12... She doesn't do sport still but loves her walks, and still tends to walk fast - still hasn't got the time management under control!
She has never dieted (in the way I would) and she loves her food. She used to have a packed lunch at school. Always two sandwiches (and I mean the four slices of bread before that debate starts), a bag of crisps and piece of fruit AND a chocolate biscuit. But when she went home she'd eat all or a bit of her dinner according to how she felt - sometimes she'd just ask for another sandwich or a bowl of soup as she wasn't at all hungry. If she was full, she stopped eating. Her mum used to serve up everything in dishes so everyone could help themselves - none of them are overweight and never have been because they stop eating or add more food according to their needs. And they had a dessert every night - could be a piece of cake, could be a sponge and custard, could be yoghurt.
My mum served up on the plate and I tended to eat what was in front of me. So I never really learned the 'that was nice, time to stop' that my friend lives by.
Now, as she's in control of her own food, she has a plus and minus approach (as she calls it). So if she is heading out to a nice coffee shop or having a day out she'll skip breakfast. Then she can have cake with her coffee or an ice cream at the local beach or whatever. They sort of cancel each other out and the ice cream is more important to her than cereal and toast! If that trip sneaks up on her (an unexpected McDonalds with the grandkids for example) she'll skip breakfast the next day and only have half a sandwich that day for her lunch.
So she doesn't diet as in watching what she eats, never has a McD, won't have a second glass of wine. Her approach is more that she watches how she eats with checks and balances.
And at Christmas, or if she is on holiday or away for a weekend with her DH she enjoys good food, treats, cake or whatever - no holds barred. When she gets home she just does the half a sandwich for lunch and half the portion of potatoes/pasta or whatever for a week. It sort of balances out.
And she has never possessed a set of scales - she goes by clothes fitting.