Practice Graduated and Continuous Shading

Try doing a seven-step grayscale. A B or 2B pencil should give you the full seven steps, though you may need to manipulate it a little to get the very lightest tones, erasing lightly and reworking. For a really effective grayscale, use harder and softer pencils to get the lighter and darker shades, overlaying differing grades to get the transitional tones. Try printing out a computer grayscale to use as a reference.