I used sour cream instead of milk in an apple crumble cake recently for lack of any other options, and I'm NEVER GOING BACK TO MILK. The cake was softer, less dry, and stayed that way for far longer than when I had used milk. I've read that you can also put a little bit of lemon juice into the liquid you're adding, and that the acidity is what matters. Also self-rising flour! Lower gluten content means softer cake, often times.