Sometimes the basic hard round brush leaves your lines feeling... flat.  If you want to throw in some quick pencil texture at the end of a digital sketch, Photoshop makes it easy! This video is not a replacement for custom brushes, but it works well in a pinch.  

Note:  In the video I use the 'Multiply" blending mode, though sometimes others are more effective.  Overlay is a nice alternative depending on the value range of your texture image.