Spartacus' advice is right on. Keep your left hand at your chin and when you see the right coming, tuck your chin, twist your left elbow down and towards your navel. This will turn your tucked chin inward making the right land on your forearm or glove. As long as your glove is tight against your chin, you can take that right. It also keeps you in position to counter with a right of your own followed by a straight left.
If you DO want to counter as a lefty, perhaps the best counter is to slip your head to your right and snap your straight left. The important part of doing this, is to throw that left with your shoulder high. Your left shoulder should slap your left ear if you're doing it right. The beautiful thing about this counter, is even if you miss with your left, it will block their right because your chin will be behind your arm. Regardless, if you throw it right, it will make them think twice about throwing the right again.
Other than this, circle to your right away from the right hand, but then you have to look out for left hooks.