If you miss a left hook the best thing to do is to duck and load up for another left hook because if your opponent is looking to throw a left hook to counter you with it and you make him miss then you're in perfect position to land a counter against him.

One thing Rocky Marciano sometimes did was to follow his left hook with a right uppercut because to avoid the left hook you ducked under it and if you did that vs him you duck right into an uppercut. Also a big right cross wouldn't be bad to follow up with either.