Here are some examples I found of where I have tried to use secondary action in animations I have made.
In the one below I tried to use subtle extra movements to emphasize the effect of the primary movement. For example when the character is pulling his leg back to kick I made him also pull his arms back to emphasize the turn of his whole body and the effort he is putting into it. I also tried to include some expressions, but some of them are poorly staged in this animation. I thought this was still appropriate to show though as it's good to see examples of both good and bad uses of the principles.
I think that the times when the expressions come before or after an action (for example his rolling eyes after he has been swung violently to the side, or his look of confusion before he falls) work quite well, but other times when the expressions come at the same time as the actions (for example the look of fear when he cowers from the hand, and the look of shock as he is falling) the expressions are completely lost and cannot be seen.
Another example of subtle secondary actions - the curling of the tail and the flattening of the ears emphasizes the stretching pose: