Finding out that someone has used a key ingredient you bought for a specific dish is frustrating, even worse when "someone" was you. Designate a "recipe corner" in your fridge to stop your family or housemates from using up ingredients you need for a meal later in the week. When you get home from grocery shopping, put everything you need for your recipe in one corner of the fridge. If you have more than one, just try to keep everything together, and within sight so no one will mistake them for up-for-grabs food. You can even use a bowl or container to group the ingredients together so they're easy to pull out of the fridge when you're ready to cook.
If you have items that don't need to be stored in the fridge, designate a recipe corner in your pantry to keep those items separate. Of course, the key to this method is making sure your family or housemates (and you, of course) know to stay away from anything in the recipe corner.