Whether you're building an addition to your house or decorating a cake, it's important to think Otherwise you end up covered in icing, which is delicious, but not the most productive use of time. I always draw sketches of my costume concepts, and often I go through several drafts before I'm happy with my design. Sometimes I even post the sketches on Facebook and ask my friends to help me decide between options. These were sketches for my Purple Velvet costume with silver vines.
Here's a preliminary sketch I had made of my Smoke Costume. I played with different ways of doing the back, and used scribbles to simulate beading. It's not always necessary to be very detailed in your sketches - even a scribble can help you determine if you like a concept or not.