Being from Pittsburgh, overcast days are common in my life. Learning to visualize your final image and make the appropriate adjustments to exposure, development, and printing is something that's worth working on.

You'll never be able to make a gray day look like a sunny day (at least not without a ton of darkroom work...just the lack of sharp shadows alone is enough to keep you fromt easily emulating sun), but on a gray day you can still produce photographs with a lot of life to them.

If you haven't read them yet, I can highly recommend "The Negative" and "The Print" by Adams, as well as "Creative Black and White Photography" by Les McLean as good books to help you with visualization. There are countless other good books out there as well that delve into this subject. (Besides, on days when you feel it's too gray to make photographs, reading about photography isn't a bad option!)