Build your colors gradually, and layer them. Apply a light undertone, and build darker colors slowly on top. Crayons can actually be more difficult to layer with due to the high wax content. Don't confuse them with colored pencils! If you want the "look" colored pencil, just use that instead.
I think the biggest problem people have when drawing with crayons is that they regress to being five years old again. They hold the crayon like they're coloring again, instead of applying it like a fine art medium. Another problem is that many people want to make it perform like colored pencils. While similar in application, they are quite different. You should not use crayons and expect them to behave like colored pencils. You'll be setting yourself up for disappointment.