Good advice all over this page. I would add that, by the time of the Peloponesian War, the Greek phalanx was having major issues dealing with light infantry. Cavalry was never more than a way to pursue already fleeing hoplites.

Accordingly, if you are interested in playing with an eye toward history, an effective method is to load up on light infantry, such as (if you are Roman) velites. You really only need one cavalry unit. I have used this method on both h and vh, version 1.5

Position your heavy infantry as far away from the anticipated enemy position as possible. Position your velites as close to the enemy position as possible.

Make the phalanxes (phalanxi? phalanxen?) chase you all around the map and keep hitting them with missile fire. Whenever possible, seperate a single unit from the line by making it chase you, and then hit it with missiles from both the front and the rear at the same time.

Anytime a phalanx unit flees, immediately send cavalry to kill whatever is left of it.

Don't worry about the line advancing and meeting your heavy infantry. This will eventually happen, but in the meantime you can do significant damage with the velites.

When the velites run out of missiles, have them retreat either behind the infantry or withdraw completely off the map. If you handle the withdrawal carefully, you should be able to use each velite unit to lure a phalanx unit into chasing it. Just stay close to the phalanx, but don't engage. Done properly, you can ensure that when your velites withdraw from the map, the phalanxes meet your line of heavy infantry in uneven fashion rather than in a solid line.

Make sure your heavy infantry (again, if you are Roman), toss their pilas just prior to contact while the the phalanxes are still chasing the velites.

Hit each phalanx with two units of heavy infantry- one straight on and, about an instant later, one from the flank. The enemy line should have been broken up enough by the velite-chasing that you have enough units to do this (some phalanx units won't have reached you yet). If not, then do this to the phalanx units on the ends of their line and leave the ones in the center for later.

Again, when a unit breaks have cavalry hunt it down and kill what remains.

As each phalanx unit breaks, you should be able to continue to hit remaining units two for one. You will win.

There are a lot of ways to win, and a lot of people have posted good tactics on this page, but I like this one as it plays into history and does provide the AI with a chance to win (sometimes they don't take the bait during the chase and then its a hard fought close one that could go either way). Good luck and good hunting.