I've never been on a diet - ever. I think diets are far too short term and revolve around depriving yourself of something which then makes you think about it even more and obsess on it. I prefer to eat healthy food 90% of the time and junk occasionally.
I also use any excuse to exercise, and I don't mean going to the gym either (can't think of anything worse) but I bike to work in nice weather, walk the dogs, go for a stroll at lunch time weather permitting etc
I did find myself putting on weight after Robert and I moved in together and found it was portion control that was my issue. Robert would cook the meals and then serve me the same size portion - being a well brought up girl, I'd politely eat it. Simple answer - smaller plate for me.
I don't have much willpower with food so I don't buy stuff at the supermarket like potato chips and chocolate biscuits - I'd eat them. I buy biscuits for Robert that I don't like. I hate ice cream so can happily have a tub of it in the freezer for Robert.
We also eat wholegrains where possible - bread, pasta etc. We ensure our meals at least 50% veges/salad and keep fat/dairy to a minimum. We also have to ensure we get enough protein as we are vegetarian.
This is a bit of a hot topic for me at there is a girl at work who often ponders why she can't loose weight and yet most mornings she sits and eats snacks out of packets - chips, popcorn etc, eats takeaway lunches, drinks canned soft drink most days and then more packet snacks in the afternoon. She declines if invited to go for a walk on a nice day at lunch and drives to the mall to get her lunches (on the rare occasion I buy lunch, I walk there)... she joined a gym but I don't think she goes very often. She is her own worst enemy

Mind you, she also comments "how lucky!" we are to travel - its called saving but don't get me started on that