Drawing with crayons is a usual child business, isn't it? But I tried them again, not long ago and can say that very interesting effects can be made that way, and it's really worth trying.
So there are some tips from me if you are eager to try:
1. Make a simple sketch with a lead pencil, outline it with pen and then use eraser to remove all pencil lines. Otherwise it may spoil your colors when you start coloring your sketch
2. Place light tones and leave blank spaces for highlights. For example, if you draw leaves, you should color their shapes with yellow color first and only then place green above.
3. Place darker tones. For leaves colored yellow it will be light-green for highlighted areas and darker green for shadows. It's like making layers in Photoshop, but strictly from light to dark, and you're unable to correct most of the mistakes on paper.
4. Add more shadows if needed (with black pencil) and highlights (with eraser).
Non-child-made crayon pictures look unusual and nice. Sometimes, it's good to make some difference and take a rest of computer art.
Besides, such pictures sell. Three of mine were bought soon after approval.