Mud guards for your bike are a must if you're going on a ride where you might experience mud or rain. If your bike doesn't have them, it's easy to make your own from a laundry detergent container.
The Instructable through the source link has all the details, but you're basically cutting off one side of a large laundry detergent container, and attaching it to your bike frame above the wheel with a spider nut or elastomer.
The curvature of the container should be perfect to keep mud and dirt from flying onto your clothes if you ever get caught in the rain. We've shown you how to do this with an old wheel, but this option is far cheaper, though admittedly not as attractive.
Cool Bike Fender For Free [Instructables]