How to be assertive with a naughty 5 yr old!! He takes me for a ride. Shouting or showing aggression is not really my style.

Agree that kids at that age are just "testing waters" to see how far it would take to break you or for you to give in to their demands. One thing I learnt with my 7 year old daughter when she was younger was never to reprimand her in public or anywhere in front of someone else. I'd always remove her from the situation immediately and bring her to a spot where no one can see us before telling her what she did wrong or if it's something that she had done before, I would make her sit and face the wall to reflect before leaving her alone. My instructions would be not to move her butt off the chair. After 10 mins, I'll come back and ask if she knows and she would explain to me. And most importantly, if she gets it right, I would ask her to kiss and hug me before making a promise to never do it again.