It's mainly your diaphragm you use to push out the baby and you use that everyday to breathe with. My friend plays the trumpet (or the trombone, i forget) and she had a very strong diaphragm but it didn't significantly shorten her 2nd stage with #1.

If you want to practice the "pushing" you can do so on the loo when you're doing a number 2, it's in one of my books, hang on.....

Right, it says to round out your spine (so don't sit bolt upright), put your knees wide apart and up on something if you want to (to be in a more "squat" position, i used to put mine on the upturned baby bath), now push when you feel like it (when your body tells you to) but DON'T STRAIN or push for a long time or hold your breath while you push, just push the normal amount for that function. Try to concentrate on pushing "from above" and keeping your stomach muscles relaxed. There's an analogy about pushing a marble through a smarties tube with your finger(do you have smarties? little chocolate sweets in a cardboard tube), if you squeeze the tube AND push down on the marble it's hard to move but if you only push down on the marble it moves a lot easier. Practising the relaxation of the abdomen while pushing is the focus of the exercise, NOT pushing hard.

In labour the most important muscle is the uterus and it's toning itself all the time with braxton hicks. Drinking raspberry leaf tea in the last few weeks can help tone it and allow it to be more "organized" and thus effective. I drank the tea only sporadically and was told not to push for such a long time that my uterus got DD out to her ears before i pushed.

When you get to that bit of labour your body will tell you what to do.