It is important to be ready and aware of how to
defend against a roundhouse punch because it is a common attack.
There is less room for error than if say your
attacker engaged you with a hold because a roundhouse punch
happens faster and is more dangerous. The technique that you
would use in such a situation should be simple and effective
so there are less errors that an adrenaline rush could cause
you to make.
There are tell tale signs that a roundhouse sucker
punch is starting to launch at you. Watch the threatening persons
shoulders. If one starts to drop you can be sure a punch is
preparing to come your way.
If I was confronted with the realization that
I was about to be suckerpunched, I would step back or forward
into the punch, depending on the location of it, with my foot
on the corresponding side and raise my arm to block the punch.
Then grab the offender's arm while simultaneously striking his
face with my opposite arm.
After a punch or palm heel strike to the face
your extended arm can then be quickly used to grab the back
of his neck and draw his head down for a knee to the groin,
stomach, or face.
This maneuver is demonstrated much more clearly,
along with many others, on Street
Self Defense 101.