Scenario: because you absently drew it on notebook paper, your clever image is forever marred by horizontal blue lines, so you upload it as-is. The only thing you can do to remove those lines would be to digitally white them out, one by one, right? Wrong, there's a quick easy way to strip the stripes off your picture.

In Photoshop* go to Menu > Image > Adjust > Hue/Saturation. Select blue, and drag the Lightness dial all the way to the right. Now all the blue in the image has turned white, including those lines. Repeat with cyan to remove them completely, maybe green if it still needs it. Do it with magenta and red if the vertical line is in the image too. Done! Your picture now looks like it was never drawn on notebook paper. Wasn't that easy?

You can also go to menu > image > adjust > brightness/contrast and tweak the controls there. This can make it so that the wrinkles in a crumpled paper will vanish, accidental pencil lines disappear, and the remaining grayed-out ruled lines go away completely. It's essential to tweak all your scans with brightness/contrast, even if they weren't on ruled notebook paper, to make them look sharp and clean.

There, all done! Wasn't that easy? You didn't have to white everything out by hand, it's all automatic!

* I'm using Photoshop, but I believe this can be done similarly in other digital art progs, including free programs such as the GIMP, Picasaand the older version of OpenCanvas. Experiment, and tell me the results so I can add that information to this tutorial, which will help more artists.

Note: this only works with black line drawings, with pen or pencil. If you used many colors, this will not work for it.