I'm a little proud of this little technique as I came up with it myself.
When cleaning up scanned black and white line work in Photoshop sometimes you miss small spots and gradients that you wouldn't see on the screen but show up when printed. Here's what you can do so you don't miss any spots.
First, you need to separate the black lines from the white background. Do this by going into Channels. It doesn't matter if it's CMYK, RGB or Grayscale (I prefer Grayscale) Just hold Ctrl and click the very first channel. You have now selected everything that is white so the next step is Select-Inverse (Shift-Ctrl-I). Now you've selected everything that is black.
Now create a new layer and fill the selection with black. You now have a layer with only black lines. At this point if you were in Grayscale please convert to CMYK or RGB color.
This is where the neat trick comes in. Add Layer Properties-Stroke. (The little fx symbol on the Layers palette)
Any little flecks or scratches you missed now have a red blob around them which makes them much easier to spot. Just erase them and now you have much cleaner art.