Mine is on average 220-240. I have been trying to implement some changes which could potentially bring it up to 300+. No stamina for that many games though lol.

Its not merely practice. Its Specialized practice with the intent on improving the speed in which you execute your tasks as quickly and efficiently as possible.

I used to use a map called Hotkey trainer in Brood War. It helped me to build some stamina and momentum to go up to 360 APM. I transferred to ingame of 240 by then due to the need to think (Training map doesn't need any thinking lol)

Once you practiced a bunch, then you put it to the real application.

FPVODs help tremendously as the mouse cursor can reveal to you where your APM stagger occurs the most.

Example from my youtube channel is here from a WOL game.

http://www.youtube.com/watch?v=bGgAsG9ICrY

Precisely. Momentum is an important thing that should be maintained. Getting harassed and distracted can easily break this momentum and cause confusion.

Some people do change their hotkeys but I don't since its my habit since BW (All defaults). I only added the Camera keys, pretty much else is stock .

Positioning and grip of your hardware can also provide an effect to your APM. That's why Flash measures his position first with a ruler .

Having lower APM than your enemy yes it is possible to win. However, if you want to do some important skills such as Terran Splitting, Zerg Creep Spread + Inject and Multiple Caster management, you do need a good amount to perform them at an effective rate.

When I play as Terran, it is important to control the army and also control as many groups of harassing units as effectively as possible at the same time. Meanwhile, you also need to macro before, during and after fights to ensure you have an army and not a bank. The more things you need to do, the more APM that is required to perform them ideally (Means no macro banking, proper control etc).