I think having two 'foolproof' looks - one for day, one for night - is key. I like to save these outfits for emergencies, aka when I'm running late. These items are most likely already in your closet, it's just about putting them together in your mind, or in snapshots like I do - I'm a visual person.
There's nothing worse than being in a rush, trying on outfit after outfit, only to end up even later with a floor covered in clothes, ever been there? The key to these effortless looks is to keep it to 2 or 3 colors TOTAL. I know whenever I am in a rush, I tend to think it's the best time to create a new outfit - not a good idea. Keep it simple, and you will look more put together. I've chosen the same color scheme for my day & night looks, simply because I love black and gray together.