One of the most important innovations of the Macedonians that helped Alexander conquer most of the Western Ancient world was the sarissa phalanx, which could plow through almost any infantry formation. The great length of the sarissas enabled the first 5 ranks to engage in combat which granted them a big advantage over similar phalanx formations, and it would completely destroy light infantry formations. Here's some old documentary footage on the subject:

http://youtu.be/fwp0vBaVgNo?t=35m33s

This documentary footage gives a good demonstration of the formations but the sarissas they are using here are only 14ft long and they barely look rigid enough to be effective in combat. This is a bit concerning because sarissas were built much longer than this, up to a length of 23ft or even more (I've heard mentions of 27ft sarissas but I cannot find the sources)! One can imagine that a wooden shaft of this length would be so floppy that it would be hard to keep its tip from touching the ground!

One of the key facts we learned in materials engineering is that wood has the best hardness to weight ratio out of any natural construction material, so we're pretty sure that wood is the right material for the job, but how can we improve on nature's design? If they could make the spear shaft hollow it would allow it to be more rigid, but it's incredibly difficult to hollow out a narrow wooden shaft. Spear shafts were a huge part of Hellenic industry and culture so we can be sure that they had incredibly skilled carpenters but did they keep any secrets on how they made their spear shafts?

http://en.wikipedia.org/wiki/Sarissa