This one might sound kind of obvious when I say it, but I can’t tell you how many times I’ve been out shooting with a group and one or more people in the group has come up and said, “Well, the sky is totally messing up our shoot today.” While a gray sky definitely stinks, there is something you can employ for shooting on gray-sky days, and that is simply to compose so little (like the shot you see here) or literally none of that gray sky winds up in your shots. If you go into the shoot knowing that you’re going to do your best to avoid seeing much of the sky in any of your shots, you can then get all of the benefits that a gray sky usually brings, which are colors that are actually fairly saturated and softer shadows throughout your images. You probably won’t be able to fully eliminate the sky from your photos, so just compose your shots so the amount of sky you do see is kept to a minimum. This simple technique has saved many a landscape shoot.
Excerpt from Scott Kelby’s The Digital Photography Book, part 2.