In the video accompaniment to Martin Evening's book Adobe Photoshop CS2 for Photoraphers, he mentions a good tool for setting the exposure and shadow sliders on the "Adjust" tab in Camera RAW.
If you choose to uncheck the "auto" feature and set your own levels, you can hold down the option/alt key while dragging the slider to put the image into a threshold display mode. This makes it easier to judge just where the highlights start to clip and what the best setting for the slider would be. Simply release the mouse to complete the action.
The same feature also applies to the "shadows" slider just below exposure.