Once you have mastered the previous dead bug exercises (see posts http://wellerbodiespersonaltraining.blogspot.com/2013/03/dead-bug-variations.html http://wellerbodiespersonaltraining.blogspot.com/2013/03/abs-right-way.html) you can progress to even more challenging variations:
1.Lie on an unstable surface such as a foam roller, BOSU, or even a pillow. Start back with the first progression (find neutral spine, maintain it while moving just the arms or legs), only increase difficulty when you can keep perfect form.
Once you are good at this, you can add a dumbbell or medicine ball.
Correct form is essential; it makes no sense to increase the difficulty if you cannot do the exercise flawlessly. Challenge yourself, but keep perfect form, and your abs and back will thank you.