Android - What is the fastest way to turn the phone into silent mode?

Personally, I think that the easiest way (apart from sliding right to left, but that's not available on all phones) to put the phone into silent mode is to hold down the power button for a short period of time, then select Silent mode.

It CAN be shortened!

You can silence your phone straight from the lockscreen by sliding from right to left.

See this video: http://www.youtube.com/watch?v=oTQ-c7QJdYQ#at=4m

Holding the Volume-Down button on most Android devices from the home screen will drop to silent mode.