I took my daughter to see Kung Fu Panda, which is one of the first movies she has seen. It amazed me that after weeks after the movie she could clearly recognize and excitingly point out the characters from the movie that were on advertisements, tshirts, and even at McDonalds.
She is two years old and she sees lots of panda bear pictures and stuffed animals around but only when she sees Po from the movie, she gets excited and tells me it is from Kung Fu Panda. The same thing happens when she sees the tiger from the movie.
The point is this, your characters have to be unique. You can use accessories, clothes, unique facial expressions, over-obvious body features, and much more. They should also all be a similar look and feel to them.
Student Examples and Comments
Combine shapes in creative ways the best you can using the pathfinder tools
Color gradients and the mesh tool to help make characters much more realistic
Try shadows and highlights to add more realism.
Choose a good color scheme that mixes well together to create harmony
Your characters should all follow a similar scheme with matching styles
Scew your characters when needed so they are more realistic
Use various camera angle techniques for better perspective and development