The sharpen tool isn't all that effective but you should be aware of it.
With it you can sharpen pixels within the area of the brush that you use (sound familiar?). Once again you'll have brush palette options so you can choose the size of the sharpen brush.
Create another duplicate layer of the background by dragging it to the new layer icon at the bottom of the tools palette. Hide any other layers above it in the layers palette so you can see what you are working on.
Simply click and drag the sharpen tool brush around to watch it sharpen pixels.
There are other methods of sharpening that I recommend (such as the Unsharp Mask filter which you could use on a layer by first 'selecting' portion of it on which to run a filter). Here is another example of the sharpen tool at work.
The palm leaves were blurred to start with and now here it is after the sharpen tool.
You might want to use it on a lower setting so it isn't as harsh to touchup certain areas. Double click on a layername in the layers palette (or right click and bring up the layer options) to rename a layer.
It can be a good idea to name layers but that's up to you. Here is the previous tutorial on blurring which adds a nice retouch effect
and this tutorial on the sharpen tool.
