Pelting them with pila/javelins also hits morale hard. This is especially useful against Naked Barbarians and other lightly-armoured troops. They will take huge casualties and may even rout before they get into melee range.

Against cavalry: Incendiary pigs (against elephants mostly) and camels (against horses only) scare them.
Against infantry: Berserkers and scythed chariots frighten them.

I think that surrounded enemies also feel somewhat more frightened, too.

Also, if you have Arcani or some other ambushing troops, or if you are in a forest, you can try ambushing the General to end the fighting early. Still, don't count on winning battles like this with horrible troops. I was fighting a spear warband and a general (2 units total, about 150 men) with my group of 6 units of town watch (Brutii, Rome). I thought I would win the battle by completely surrounding and flanking the spear warband, but instead, all my troops just chain-routed. Also, don't be fooled by their "spears." I had a general charge uphill into a well-formed line of these guys and they flew off of the map, litteraly! They are basically almost as horrible as peasants. If you want a unit to hold something, I recommend getting half as many units of Principes or Auxilia (later in the game).

Overall, I find that killing the general is the most effective way to get routs because all of his troops are hit equally by his death. Also, the AI usually uses the excellent tactic of charging the general into a tightly-packed phalanx to start the battle off, so killing the enemy general is usually a fairly simple task.