Simply grab two towels and wrap them around the barbell where you'd normally place your hands.

This is also a great variation for people with shoulder issues. The towel allows for a neutral grip, which is a very easy position for the shoulders.

It also forces the lifter to grip with more force, thereby activating more stabilizer muscles in the shoulder girdle. Lastly, it will force the lifter to use a lighter weight, which again will be a little easier on the shoulder joint.