Haven't read thread so apologies if I am repeating anything
My top invention is a mixing breakfast, which is empty yogurt pots full of all the things you want them to eat. You put all the pots on the table and let your child choose what to put in their bowl. If they are under 3 they are almost certainly going to tip the whole lot in, and then hopefully eat it. Older ones can pick the things they like and feel like they have control. You could try this with salad contents and so on as well.
Try not to get too het up if your child won't eat "proper" foods at each meal, or the appropriate foods for that time of day. Homemade pizza for breakfast is nutritionally equivalent to a bowl of weetabix, milk and fruit juice. It's just wheat flour, cow's milk and tomatoes - perfectly good food.
Find healthy snack foods that they enjoy and let them help themselves whenever they like. Give them a drawer, box, or section of the fridge that is just for their snacks, and let them choose when and what to eat from it. Fruit, dried fruit, nuts, oatcakes, mini cheese portions, crudites and dips etc are all great foods and can go a long way towards meeting a child's nutritional requirements for the day.
Go to a greengrocers or market to get your fruit and veg instead of a supermarket. Children can see, feel and smell the produce and choose new things that they would like to try. The staff hopefully will get to know your child (regular customers at a market or greengrocers get GOOD treatment) and may make a fuss of them or give them treats when they come in.