After making the grayscale photo fit the layout, it needs to be enhanced. My favorite tools for enhancing a grayscale image are the Dodge and Burn tools. Using these tool, you can easily lighten or darken (the Dodge tool lightens and the Burn tool darkens) the image and not lose the existing textures and gradations.

To get the best results from these tools, it's important to set the options to fit your need. I usually use an exposure setting of around 30 percent and then change the Highlights, Midtones, and Shadows settings depending on the areas I want to effect. For instance, set the Dodge tool to Highlights to to make a highlight on the eye whiter, or set the Burn tool to Highlight to make the highlight on the eye darker. So, in a nutshell (look I'm in a nutshell! How did I get in this nutshell? Look at the size of this bloody great big nutshell. What kind of shell has a nut like this? This is crazy!), set to Highlights to effect the lightest areas, set to Midtones to effect the middle tone, and set to Shadows to effect the darkest areas.

If you have a graphics tablet you can check the Size or Exposure boxes, allowing you to vary the size or exposure by the amount of pressure applied to the pen.

Tip:
When adjusting a grayscale image to be converted to line art, exaggerate the contrast to get the best results.