Take the example in this post. I have about 6 months worth of data now that I can examine. In comes conditional formatting.
Say I want to look at weeks where my coupon use was above average. First highlight the column you want to look at (column B in my example). Next, under the conditional menu, select "Top Bottom Rules" and then select "Above Average". A menu comes up, and you can select the color you want to make any cells that contain numbers that are above the average.
Done! Isn't that helpful?