Local adjustment brush is a great tool in Lightroom 2 that makes photographers use Photoshop less frequently. Earlier you could make only global changes on your photos that impacted the whole area of the picture. Now it got local.
The very same – any more – effects can be achieved with layer masks in Photoshop. However, this tool is so easy and intuitive to use in Lightroom that sometimes I have to resist using it.
I show below how it works.

Look at the pic on the right. The photo is dominated by the model’s eyes and smile and the eye really attracts the attention. However, we could be kind to our model just making her wrinkles just a bit less visible.

The tool is available in the Develop module as brush icon under the histogram on the right hand side panel. Clicking on it will reveal the adjustment options you can chose from.
They are: Exposure, Brightness, Contrast, Saturation, Clarity, Color.
We are going to be interested in the Clarity slider, but first lets select the area to be adjusted.

Set the size of your brush and give it some feather for the smooth transition between the adjusted and not adjusted areas and paint the “problematic”. See what I selected in my wrinkle hunting quest.

Clarity can help in showing more details if you push the slider to the positive direction. On the other hand you can soften edges nicely when using it negatively.
I pushed it completely down. Check the result.

You can use the same tool to emphasize the eyes (Clarity to positive, maybe Exposure and Contrast to positive), whiten the teeth (Brightness or Exposure to positive and in certain cases Saturation to negative) and many, many other things.