Colin Litster has posted a tutorial on faking really nice looking volumetric clouds on his site The Cog Project. It covers working with textures on particle systems, lighting them well and then cheating a bit.
OK creating clouds in Blender is not easy so do not expect volumetric 3D particle simulation solutions in this quick tutorial. What I will give are some of the solutions that I have found that give reasonable cloudlike effects in blender. In this Cog's Quick tutorial I'm not supplying a blend file to start you off. It's a good idea to try to create your own cloud based on some of the settings shown. In this way you can experiment and create your own unique clouds. However, I do give a final blend file that you can view and explore.
You can read his tutorial here.