The most important thing when trying to photograph clouds is to simply always have a camera with you. If you have a focus setting on your camera, set it to infinity or landscape. If you have a camera that you can add filters to, you might consider a "Polaroid" filter. This is like a pair of Polaroid sunglasses, and it increases the contrast between the clouds and the sky. It tends to make clouds look more dramatic.
There are lots of computer programs you can use to increase the contrast and change the colors of your photographs. These should be used sparingly. It is very easy to get carried away and make a sunset look like a Margherita pizza by exaggerating the intensity of the colors. Remember, when it comes to messing with your photos on the computer, less is more.
Special thanks to Gavin Pretor-Pinney, founder of the Cloud Appreciation Society, for sharing his knowledge about clouds. For more information, check out his latest book, "Hot Pink Flying Saucers and Other Clouds."