• A graduated neutral density filter will help you avoid over-exposing clouds.
• A circular polarizing filter will bring up contrasts within clouds.
• Give interesting and dramatic clouds the top two-thirds of your frame.
• Scarce or small clouds will look better taking up the top third.
For more tips, check out How to Photograph Clouds: Using the Sky to Enhance your Landscape Images by Paul Lightfoot at Suite101.com.