Here is a sample - before and after. I also used dodging and burning as explained in another lesson.

1- First convert a raw file twice. Once for all the highlight details. Again for all the shadow details. Like this:

2- Now drag the darker version onto the top of the lighter version. Make sure View - Snap is checked. Use the Move Tool  from the Tool Box. Click and drag. As you get close to the edge it will SNAP into place.

3- You now have two Layers. Window - Layers should be open. Activate the lighter version (bottom layer) by clicking on it.  You will see it turn blue indicating it is the active layer.

4- Now hold down Control + Alt + the Tilde key (looks like this ~). This automatically places a luminosity mask in place.  (Please note that in CS5 you will use Control +Alt+2 or 3 keys) You will see the infamous marching ants showing the area affected. It will look like this:

5- Now activate the top layer (darker layer).  Add Layer Mask by clicking on the third symbol from the left on the bottom of Layer Box. (It looks like a gray square with a white circle in the center.)  Like this: Look for the white curser arrow. 

7- The rest is easy. In the top layer click on the mask. It is the one on the right side. Now add a little Gaussian Blur to the mask to blur the details. You can see the results as you go (Filter-Blur-Gaussian Blur).  Be careful not to add to much as you will get a halo effect. Many photos will not require ANY Gaussian Blur. Also, and this is important, erase that part of the mask you don't want using the eraser tool. Experiment and you will see what I am talking about.

8- Now feel free to touch up your image as you see fit. By using the Dodge Tool on the mask itself you can make any portion darker - like the sky part. By using the Burn Tool  on the mask  you will cause the bottom lighter photo to come forward. Add a touch of contrast and saturation and you have this. 

Another use. Notice the windows. Click to enlarge. Look for lower right hand arrows to make even larger.