You've heard of Deke McClelland's Photoshop Bible? Here's a page from Raptor's Photoshop Dead Sea Scrolls!
"File -> New
For horizontal scanlines, make an image 2 pixels high, 1 pixel wide.
For vertical, make an image 1 pixel high, and 2 pixels wide.
In either case, pick a transparent background. Now get your zoom tool. Zoom ALL THE WAY in. Pencil tool, and fill one pixel with black. Now Edit -> Define Pattern.
Now go to a different PSD, and click Edit -> Fill. Set your options:
Using: Pattern
select your pattern
Mode: Normal.
When you're done with that, try adjusting the opacity/changing the layer blend mode of the layer with scanlines on it."