Like pullovers? Ditch the bench and use a foam roller instead. Besides promoting optimal spinal alignment and shoulder mechanics, the instability of the foam roller forces you to use strict form and eliminate momentum.
The foam roller has a tendency to move unless you remain tight and lock in your core. This creates significant rotational forces that you must resist to keep from falling off the roller. Any wiggling, cheating, asymmetrical movement, or shifting will cause you to lose balance.
Pullovers are already known for their effectiveness in targeting the abs, but this variation takes the core activation up several notches.