Trulia recently came out with a fantastic guide to buying bedding, including duvets, pillows, and sheet sets.
Here is just one great section of the article:
A tip from the author: “Not sure if it’s time to replace your pillow? Here’s how to find out: Fold the pillow in half. If it springs right back, the pillow is still in great shape. If it doesn’t, the pillow needs to go.”
You can find the whole article here.