Stepping in quickly with a fast hard jab after the slip can stop an opponent in their tracks before they can get a right off. To do it effectively after the slip, roll quickly pulling your right shoulder back, and then snap in a left jab violently while simultaneously pushing off of your back foot. You gotta keep it short and quick. The only thing on your opponent's mind is, "It just happened so fast!"

Otherwise, get ready for his right. Block it with your left glove, or check his right bicep with your palm before he can get the shot off.

Anyways here's some good posts by grey, different counters off the slips and more:
http://www.saddoboxing.com/boxingfor...-slip-jab.html
http://www.saddoboxing.com/boxingfor...g-punches.html
http://www.saddoboxing.com/boxingfor...-off-slip.html
http://www.saddoboxing.com/boxingfor...left-lead.html