Make certain your front hand remains above their front hand (their jab hand) as much as possible - this will allow you to knock the jab down when they throw it - it may be wise to cheat away from your face when you (an orthodox boxer) is facing a southpaw - Your left hand should be above their jab hand (their right hand) - Granted, if you yourself are a southpaw and you are facing another southpaw then, of course, it will be your left hand knocking away their jab (their right hand).