When editing my images in PS I typically create mutliple adjustment layers. As you can see in the image below I have the original (background) image layer, a levels adjustment layer and a curvers adjustment layer.
When I finished making adjustments to the image I almost always need to sharpen the entire image using either one of the sharpening filters or by using the High Pass filter (Filter > Other > High Pass...) which is my favorite. In order to apply the sharpening filter to the original image plus all the adjustments the multiple layers must be combined into one layer. This is done my pressing CTRL+ALT+SHIFT+E which will create a new top level layer that is a merge of all visible layers.
Now you can select the new layer and apply your favorite sharpening filter to the entire image. 
