Its a really tough habit to get out of, but DO NOT button mash your Instant Cast macro while you are casting the previous Lightning Bolt in anticipation that you will kill your target with it.

You should be 90% sure your target will die with the Instant Cast macro, and you can't have that sort of certainty while you are halfway through casting another spell. If it is VS a Warrior that is not in Prot stance or a Hunter, you can usually be sure of his death earlier on, but VS any class with a get out of jail free card, you must cast lightning bolt to soften them up, take a half second after the cast to assess the situation, and then make the decision to burn your Instant Cast macro. At this point, the only thing preventing your targets death is his reflex of using his Fail Save VS your Instant Cast of Death.