Use good quality crayons (Crayola works well) for the best results. Making the fill color darker around the edges is a good technique for adding dimension. As you work, don't press too hard. It's better to add layers of color than to have too much color that you can't remove. Take care to color evenly and without causing color streaks. When you're finished, place a piece of scrap fabric over the coloring and iron the paper on a high setting. This will heat set the coloring and remove excess wax.
Here are some tips on using crayons for effects in coloring: You can start with a light layer of the color you want to use first. Then you can cover that with darker or lighter color to add life to your drawing, just like the wings on this butterfly.