Meal planning, especially when dieting, is like a game of checkers or chess - you need to be thinking a couple moves ahead.
Sure, there are times when you'll need to be impromptu, to adapt and overcome when life isn't going particularly right on a given day, but make no mistake, winging it and weight loss don't go together over the long term.
When you're making dinner, cook more than you'll need, then you'll have something to eat for lunch the next day. When you're making a mid-morning snack, think ahead to what you'll snack on that afternoon and prep it right then and there. Doing little things like that can help overcome whatever might interfere with your day.... You're already prepped and ready because you were thinking a couple moves ahead.
Another way to meal plan is to prep a bunch of meals for the coming week. I love to grill, so on my days off from work, you'll probably find me lighting the charcoal, not for just one meal, but several. I don't just grill a few things, I grill in big batches, why not, the charcoal is hot! I'll grill up chicken done a couple of ways, some pork chops done in different marinades, a few burgers, perhaps some fish.... Some of that will go in the freezer as a base for future meals, some of that will go in the fridge for the upcoming week.
A good way to keep variety in your diet is to just alternate your days with different proteins. What do you feel like, beef, chicken, pork or fish? Start off with what hits you first, then alternate, pairing proteins up with a complimentary veggie. Grilled chicken and broccoli, pork chops and asparagus, steak and mushrooms, fish and green beans.... There, boom, four meals!
This isn't for everyone, but I have a white board in my kitchen, for menus and a grocery list. The point here though, is to jot ideas down.
Look in the freezer, peak in the fridge, check out your pantry and see what you have. Start writing it down in meal planning fashion, if you don't do this kind of thing already, you'll see how really easy it starts to flow - it's easier to craft a shopping list this way too! It's thinking a few moves ahead - checkmate!