This is one of the few bits of Sheldon's wisdom that I wholeheartedly disagree with.

Using both brakes is best! Push yourself back in the saddle a bit for firm brake application.

Get your ass off the saddle and over the rear wheel, with the saddle's nose under your sternum for really firm applications and on especially steep declines. I don't usually have to resort to that on my road bike but I think it was Cadel I saw do it on a downhill in the TdF this year. I do it all the time on my MTB.

I like to keep my rear brakes detuned a bit so that I don't lock the rear so much.