The easiest way I've found is this:
1) Draw a white (or gray) colored square larger than the area you'll be mapping. The county that you are interested in should be beneath this square.
2) With the white square selected, go to 'Drawing' menu on drawing toolbar and choose 'Convert Graphics To Features'.
3) From analysis toolbox, search for 'Symmetrical Difference' then apply the tool to the county layer and your newly made square-shaped layer. This will create a new layer that is your white square layer minus the shape of the county; ie. a square mask with a hole that is exactly the shape of the county. If you move this square mask to be on top of all other layers, everything will be covered except features within the county that will show through the hole.
4) With this new mask layer placed on top of all other layers, right click on the mask layer and choose Properties > Display. Adjust transparency to reveal the layers underneath.
5) Adjust the transparency, and also adjust the colors of other layers to achieve the effect you want.
6) If you have labels, they will show above the mask. The only way for labels to go under the mask is to change them into annotations.
This is a neat trick to give emphasis to features within a county, city or other polygon, while allowing features outside to still come through. I like using this to give context of surrounding area and road network.
(I know this is replying to an old thread, but might be useful to someone else. I'm also posting here as a requirement of a MS-GIST course...)