Try to imagine a "triangle" balance point. Once you're "locked in" the correct position, it should only take minor adjustments to stay in it.
I know it's a geeky way to think about it.... but to balance a triangle on a point, the triangle doesn't have to be evenly shaped. The center of the triangle's mass is called the centroid. http://en.wikipedia.org/wiki/Centroid In terms of a manual, your hands are further in front of the rear wheel than your ass is behind it. But, that's okay. The center of your body's weight can find a spot where the "centroid" is directly above your wheel axle. You will be balanced. You adjust the position with both your hands and feet. Your feet can push the bottom bracket forward or let it come back, your hands can let the front end drop, or pull it up...
I added the "triangle" to the LeeLikesBikes.com graphic:
I've owned sooo many p3 s I ver the past 10 years... Easiest bike ever to manual. The only bikes I have remote issues with manualling are 24" jump bikes, and only because with the lower frame, it's generally awkward getting your body down that low....at first(but it's really dependant on chainstay length) Remember all the points above and mainly DROP YOUR HEELS.... If you don't your manual won't last long. Wear the stickiest shoes possible and drop your heels when you settle in to the manual.