If it helps, its not so much the stomach muscles you use to push, rather its exactly the same as if you were doing a poo!

Also, when you get the urge to start pushing you can't relaly stop it anyway!

I'm not sure about how to strengthen the muscles, but keep in mind your body was designed to push out a baby. In the end its not so much about strength as it is listening to your body...

Hope someone else can help you out with some answers