In Blueprint's tour of Amy Butler's home (from their 2nd-to-last issue), we noticed a little blurb where Amy mentions sewing envelope cases for her throw pillows. This way, she can mix up her fabrics and change them out when the mood strikes. We found some very simple instructions for making these at About.com. You don't even need a sewing machine if you have some time and basic hand-sewing skills...
How to make a basic envelope pillow case:
• To measure fabric, double the size of your pillow and add 4 inches. Example: for a 12-inch pillow, cut your cloth 12 x 28 inches. (You don't need to allow for a seam since the slip should be a little smaller than the pillow to keep the fabric taught.)
• Fold the edges of your fabric 1/4-inch once, then fold over again to create a clean hem. Hem the edges of your fabric.
• Turn your fabric inside-out and fold it into the shape for your slipcover, allowing a 3 1/2-inch overlap of fabric on the back side of your pillow. Pin the edges together.
• Use a 1/2-inch seam to stitch the open edges together.
• Turn the slipcover right side out and insert your pillow.
For detailed instructions, including how to mark your fabric, visit About.com.
Image from Amy Butler's Midwest Modern.