Philip made adjustments to the organisation of the Phalanx and developed a double ended sword called a Sarissa; definitely a selling point for the Macedonian Phalanx. Bare in mind, Alexander was following his father's footsteps as King of Macedon, so whatever he did would have had to have been better than his Father's achievements. Philip encouraged him constantly, and also think of the incident with Bucephalus. He told Alexander that Macedon was too small for him and he needed somewhere bigger to rule.

I'd definitely say that his reforms to Macedon and the phalanx were the best bits of groundwork for encouraging Alexander and his successes. Also, another thing to point out is that Alexander had many of his father's generals working for him too, so experienced men like Parmenio/n for example would have increased his chances of winning battles. Alexander also took part in the Battle of Chaeronea when he was still a teenager, and controlled one of the phalanxes - so he was on a good run anyway.