For the those of you who are traditional artists and use a scanner to your drawings, you may experience your drawings being over exposed or washed out. While it varies from scanner to scanner, here is how to correct that problem, even if you don't have any kind of photo editor on your computer.
The first thing you should do is make sure the lines of your drawings are nice and dark. I would almost suggest going over them with a pen or sharpie, but that's entirely up to you.
If that doesn't work for you, then the next step is to edit your drawing. For this tutorial I will be using Pixlr (the full version, not the one-click effect ones)to demonstrate.
http://pixlr.com/editor/
Since I don't have a messed up drawing to work with, I messed up one of mine on purpose.

Go to adjustments> brightness & contrast. Adjust the slider until your drawing looks better. And depending on your drawing, you may need to re-apply this effect more than once. I did, but that may have been because I messed mine up on purpose. But in the end you should have something that looks like this.

If this hasn't worked out well for you, then I would recommend (the proper way) using the levels adjustments slider. You can get to that by going to adjustments> levels... A box will come up that has three sliders on the top, and two at the bottom. You don't really need to worry about the two on the bottom, the top three are the main ones. From left to right, the sliders go from black, grey and white. What these mean are the shadows (black), mid tones (grey) and white for highlights. Move these sliders around until your drawing becomes more visible.

That's it! Your drawing should now not be washed out or over exposed!