So you have some line art and you want to layer it. Cool.

Should your line art not be as contrasted as you like, check out:

Image -> Adjust -> Levels

and play with the sliders until you you get the amount of contrast you like.

Anyhow, here's my ready-to-layer line art.

So now head on over to the channels pallet.

Both greyscale and RGB work for this. Just remember, don't flatten when you change to RGB mode later.

Anyway, hit the little dotted circle on your channels palette, which will select the white parts of your image fully, the grey parts partially, and the black parts not at all.

By the way, this method works in Photoshop version 4-7.

Here's the current loaded selection.

Now, either hit CTRL+Shift+I or Select -> Inverse.

This means your blacks will be fully selected, your greys still partially, and white not at all.

On your layers pallet, hit the little button that looks like a piece of paper with a folded corner.

This makes a new layer.

Now, CTRL+Backspace will fill black in all your fully selected areas, grey in partially, whites not at all.

Deselect, then go back to your background layer, hit X again to switch your colors back, and CTRL+Backspace to clear your background.

If you like, go back to your layer with the lines and rename it 'lines' or whatnot, and hit the Lock / Preserve Transparency box.

This means anything on the layer is locked so you cannot make new marks; only work with what is there. (You can even color your lines later, if you like.)