Step 1
   First create a new image 360x60
   pixels. The background can be
   any color you want it to be for
   this tutorial.

Step 2
   Now choose an image (it can be
   any image you want) and hit
   ctrl+c (copy). Now to get the best
   most visible results use a bright
   image (the scan lines will show
   up better).

Step 3
   Now go back to the image that you
   made. Hit ctrl+v (paste). Center the
   image that you just put in.

Step 4
   Now hit shift+ctrl+n (new layer).
   Next comes the tricky part...
   Select the line tool, and select
   from the file menu
   Window | Show Options then in the
   options window, change the
   following options ......
   Weight: 1 Anti-aliased: Not Checked
   Arrowheads: None Checked.

Step 5
   Now draw 2 lines straight across by
   holding shift as you make the line.
   The lines should be at the most 1
   pixel apart.

Step 6
   Select the lines and nothing else.
   Now select from the file menu
   Edit | Define Pattern. Now hit
   ctrl+a (select all).

Step 7
   From the file menu select
   Edit | Fill. In that new
   window change the following
   option to: Use: Pattern.
   Then click OK.

Step 8
   Time to watch the magic!
   Go to the layers control
   panel. Select the layer with
   the lines on it. From the drop
   down item select Soft Light.
   Now move the layer above the
   image that you pasted in a
   minute ago.

NOTE:
   If you want finer lines, download
   the psd of my extremly thin scan
   lines. Photoshop 4 users can't
   view the psd because it is in
   Photoshop 5 format.. Sorry.