Practice in color theory by applying different color schemes; monochromatic, complementary, analogous and tertiary.
Practice in tints, tones, and shades; seeing how they effect colors and the overall image.
Practice in value and contrast; how to balance the values in an image to ensure the overall composition is clear and readable.
Practice in shape design and creating an interesting composition using only a few images.
Practice in stippling and dot art; what areas contain more/less detail depending on the lighting.
Practice in line work and how line weight and shape can affect the overall image.