Thanks mate!

The Missile troops tend to go in front of the army at the beginning of a battle and have the goal of damaging the enemy's troops as they approach. Quite often, however, you'll find that the opposing general has exactly the same idea and the result will be an archery duel. This may last for a long time, either until one side's archers have been decimated and get pulled back or destroyed (in both cases, the losing player will generally be forced to attack to prevent his troops from being shot to pieces), or until both sides run out of arrows (in which case the advancing tends to take place on both sides).
There is a reasonably wide choice of archer units, especially for Eastern factions. Generally speaking, the more expensive archers like Trebizond Archers, Saracen Archers and Crossbowmen will be stronger against enemy melee troops, but for the archery duel, the cheap standard levy archers can do just fine. Therefore, it's good to have at least a few of these. Depending on how much damage to the enemy army you expect your archers to do, you can invest in a large force of expensive archers (these should hopefully beat the enemy in an archery duel and be able to inflict significant damage to his troops, you'll have less money for you melee force, though). Or you can just go for a small force of levy archers, just about enough to keep the enemy archers at bay for a while and to use up their arrows.

The front lines consist of regular infantry. These troops tend to be spear-armed and with a good defense and morale value (i.e. able to hold off the enemy for a while). A few examples: Men-at-Arms for Europeans, Skutatoi for the Byzantines or the sword-armed Saracen Infantry for Muslim nations. Thematic infantry and feudal sergeants can also fulfill this function, as long as they're slightly upgraded. Like with the archers, it's up to you how much money you want to invest in defensive infantry. Be aware, however, that (with the possible exception of Saracen infantry) their goal is not to rout the enemy, but to keep him in place for you to flank him.

Behind and/or to the flanks of your regular infantry (sometimes even to the front, so as to cause severe damage to the enemy before the onslaught of your main army) are the irregular infantry. These troops will tend to have a lower defense, but a high attack rate. Often, they will be axe-armed and particularly dangerous towards armoured troops. A few examples are Axemen, Swabian Swordsmen, the Varangian Guard or Ghazi Infantry. Upgraded woodsmen can also do very well in this function. Depending on how good they are at defending (Ghazi Infantry and Woodsmen have quite a low defense rate), you may want to keep these men out of the way of archers and cavalry and only send them in to fight on the flanks.
The puropse of these units is to attack the enemy in weak spots like the flanks or gaps in the line. Depending on the type, I've been known to use them as a general reserve troop as well. It's best to only commit these men when the enemy is engaged, as they tend to be quite vulnerable to attack.

Your final troop type is cavalry. Their function is also to attack at the flanks, as well as to engage enemy cavalry. They don't tend to be able to hold out well in a melee against infantry, though. Some of the lighter cavalry units can be sent against missile troops during the archery duel, so as to turn the tide. Only use cheap horsemen for this, as it can be quite dangerous if it doesn't work out.
Among your cavalry you can also have some horse archers. These are excellent at forcing the enemy to attack, as well as for shooting at enemy foot archers from the flank during the duel (be careful that the enemy archers don't get a clear shot at your HA, as foot archers are stronger in a direct duel). HA can also be used to lure enemy cavalry away from the main action.

Basically I make these separations between my units:
Skirmish Infantry (archers)
Main Defence Infantry (spearmen)
Aggressive Infantry (swordsmen)
Skirmish Cavalry (horse archers/light cavalry)
Heavy Battle Cavalry (feudal knights)

Non progredi est regredi