Head movement, lots of it! Or pop the machete out of the shorts and slice that nucca's arm off real good.
From another thread: Ducking down and throwing an overhand right JMM/Salido-style also works, as well as your basic slip rightwards + right hand counter. Mosley was ace at setting that up in his prime. If your opponent drops his left hand frequently, just slap a left hook onto him.
What Bogo said. Ward v Froch is a good example (if I am remembering correctly) of using footwork/slipping to take away an opponents jab.
If you're shorter, then lots of head movement, feints & lateral body movement to draw the jab, slip & counter - watch haye-klitschko