Wayne B says:

I've found that the secret to backing a sailboat is to give it a quick
burst of reverse power while still in open water. Get enough speed in
reverse, usually 1 or 2 knots will suffice, that you have water
flowing over the keel and rudder. At that point, take the engine out
of gear and let the boat coast backward under its own momentum. It
should now respond to steering just as if
you were moving forward.

I find the best way is to get the "going forward" part just right, so that
reverse is used only to take way off the boat so you can step gently onto the
pierhead with your bow/stern line at the perfect moment. An admiring audience
of curvaceous females who appreciate the art never hurts here.

It doesn't always work like that in practice, however... ;-)

Steve