You jab, jab again. At some point he'll try a right over your left- pretty likely, yes? When he does you stiffen your left arm to "leverage" his right up and away from your head while you drop your weight to your right leg- tucking your head behind your left shoulder- then throw a right upper cut to his solar plexus and a left hook to his liver.
Ah, I call that one the snag-line. Its a good move but you must watch for your opponent's left hook as he has just completed the weight shift involved in throwing his right and is thus in position to sling his left hook at you. Ha I remember Rafael Marquez getting knocked down with just that: a left hook after "snag-lining" Israel Vasquez's Right hand with his jab. Forget if it was in the first or the second fight - probably the first.

this makes the snag line a pretty dangerous strategy to employ. For both guys. A messy move.