Always Color in a Consistent Direction. This is my first tip. It will make a bigger impact on the end result than anything else and any age can use it! The changes in direction, shown on the red kidney below, on the left side, are what make it look sloppy, even though I carefully stayed in the lines. Coloring in a consistent direction, as seen on the other kidney, is easier on the eyes and ends up looking very tidy!
Mid_range pencils from brands like Staedtler or Faber Castel offer softer application, which allows you to get more color into the paper. This can produce a more vibrant color and reduces the chance of white flecks of paper showing through. Softer wax pencils are also better for shading and blending.