You've compiled a big, complicated spreadsheet chock full of numbers and you want the important ones to stand out. Give conditional formatting a try - it's a groovy Excel technique that changes cell appearance based on rules you set up, like "Turn the cell green if its value is less than 100."
The Productivity Portfolio blog's got a walkthrough with screenshots and examples:
[Conditional formatting] allows Excel to apply a defined format to cells that meet specific criteria. You can use a different background color, font color or border. The goal is to make important cells stand out so you can find them easier. Excel already does some of this for you. As example when you format cells with numbers, there are options on how to display negative numbers in red.
Useful! Now I want it in Google Spreadsheets.