While it’s usually preferable to keep the backgrounds of your images relatively simple and non-distracting, for photos having a clear sky in the background you’ll sometimes wish there were some light clouds to break up the monotony.  While you could use the methods from the previous section to replace the background with a photo of a more interesting sky, sometimes it’s simpler to let Photoshop render some artifical clouds for you.  The Filter > Render > Clouds filter often works fairly well, as long as you take some precautions to reduce the artificial appearance of the result.
    The first thing you need to know is that you don’t want to actually render the clouds directly onto your background layer.  Instead, you want to render the cloud pattern into a layer mask.  The trick is to create two background layers, one adjusted so as to increase darkness and the other adjusted to increase brightness, and then to render a cloud pattern into the layer mask of one of those layers, to achieve a blending between the lighter and darker background layers.  This is illustrated below.


    In the figure above, you can see that we have three layers.  The bottom layer is the original image.  We used the Select > Color Range menu option to select the blue sky in this image and then pressed Cmd-J / Ctrl-J to copy that selection to a new layer.  We then pressed Cmd-J / Ctrl-J a second time, to duplicate the new layer.  We then used Levels to increase the brightness of the lower background layer (the middle layer in the stack) and again to darken the upper background layer (the top layer of the stack).  Finally, we added a layer mask to the top layer and invoked Filter > Render > Clouds to create a grayscale cloud pattern in the layer mask.  This allows parts of the lower, brighter layer to show through the upper layer, creating the illusion of clouds. 
    Note that you can wait to change the brightness of the layers until after you’ve created the layer mask; this will allow you to make your adjustments to the two layers while getting immediate visual feedback.  Adjusting the relative brightness of the two layers will determine both how bright the clouds are and how blue the sky is.  Note that your clouds may appear bluish, and this is often OK: thin clouds can in reality allow some of the sky’s blue color to show through.  If your clouds look too blue for your tastes, you can either continue to increase their brightness (to bleach out the color) or apply Hue/Saturation to reduce saturation of the lower background layer.  Likewise, if you find that you need to darken the sky considerably to create enough contrast between the sky and the clouds, but that this results in an unattractive color for the sky, you can use Hue/Saturation to correct the color of the upper layer.
    One trick you can use to improve contrast between the sky and the clouds is to apply a contrast filter to the layer mask.  I typically opt to do this using Levels, since it gives me more control than the Brightness/Contrast tool.  You can also apply a Gaussian Blur to the layer mask to make the clouds look smoother.  The image below shows a poor example of artificially rendered clouds.  These clouds look bad for a number of reasons, including over-brightness in some areas, excessive contrast, and an overall rectilinear patterning that appears obviously computer-generated.


The excessive contrast and rectilinearity can be fixed by applying a Gaussian Blur to the layer mask.  As you can see in the figure below, clouds rendered in high resolution in Photoshop can indeed be made to look quite smooth using an appropriate blur radius in the Gaussian Blur.
    Another thing to notice in the figure below is the existence of some edge effects resulting from an imperfect selection when creating the background layers.  The edges in this image have actually been touched-up using the Clone tool, and yet you can still see a bit of a halo around the bird’s beak, and to a lesser extent along the edge of the wing.  Similar methods to those described in the previous section can be applied to address these edge artifacts.  (Note that the dark shadow above the bird’s head is likely the result of either flash or the use of a sharpening filter).

    In this example (above), the two main options were to either use the Clone tool (as was done here) or to go back and refine the selection of the sky (possibly via Select > Modify > Expand/Contract or the Quick Selection tool).  Using a layer mask to adjust the edges was less convenient in this example, because of the way the background layers were created.  Using a layer mask to address edge artifacts could have been done as follows.  First, we would have needed to duplicate the entire image rather than just the background (or alternatively, we could have selected the background and expanded it via Select > Modify > Expand, with a wide expansion radius).  Then we could have adjusted the brightness of the new layer (to allow visual discrimination between it and the original image) and then created a layer mask and manually traced the edges with an appropriate brush in the mask.  This layer could then be duplicated (both the layer’s image pixels and its mask) to create the top background layer as in the previous example.  The tricky part is then how to render the cloud pattern in the layer mask without obliterating the existing mask.  This can be done by selecting the part of the layer mask corresponding to the sky; if you then apply the Filter > Render > Clouds filter, clouds will be rendered only in the region you’ve selected.
    Note that you can use multiple cloud layers.  This allows you increase the realism of the clouds by introducing additional variability.  Simply create additional copies of the top background layer, render clouds in the layer mask of each, and then adjust the brightness of each layer’s image pixels until you like what you see.  You’ll probably want to reduce the density of the cloud patterns in the higher layers by applying the Levels tool to adjust the brightness of the layer masks; otherwise, you may end up biasing the cumulative masking effect so that the lower layers don’t show through much at all.  The figure below illustrates a variant of this approach: in this case, four background layers were used—one upper layer with a cloud mask, and three lower layers, each of a different color (white, gray, blue).  The gray and blue layers were trimmed using a soft-edged eraser so as to show up only in diagonal bands across the image. 



     Note that the methods described here for rendering clouds in the sky can be used in other situations to add subtle texture to other types of backgrounds (e.g., smooth water, out-of-focus landforms, etc.).  By rendering clouds into a mask layer and then applying a more subtle brightness adjustment to the duplicated background layer, you can increase the perception of texture in the background without imposing a recognizable cloud effect.  I used this method in the two images shown below before rendering them onto canvas; the added texture, when printed on canvas, suggests a sponge-brush effect like you might see in a real painting.