Best Stratagy Ever:

Kill the enemy while not letting them kill you.

It's perfect...

But seriously, I like Alexanders "Hammer and Anvil" stratagy. If I am playing as Romans, have my heavy infantry line with 2 units in reserve, and triarii on the flanks. My cavalry are all massed to one side. Once the enemy engages my line, my cavalry rush behind their line and hit them directly in the middle. If I have any archers, they are shooting at the center unit that I plan to hit with my cavalry. The whole line usualy collapses after that.