I prefer to use free weights because they requires you to also engage your core muscles while you lift, as there is no stabilizing factor other than your own muscle strength. (bodybuilding-wizard.com)
Maintain a straight line from your shoulders to your heels by engaging your core muscles. (dailyburn.com)
This page has pictures of a plank variation that adds some muscle work to the inner and outer hip while also engaging the core muscles. (ab-core-and-stomach-exercises.com)