if i have a mix of complete and incomplete stuff coming on, say in a Crusade, i like to leave the very small units till last in order to keep as many men on the field as possible.

if you're fighting in the desert, try keeping some of your heavily armoured units in reserve so they're fresh for a counterattack and don't get exhausted, or keep something to replaced armoured units that do get exhausted.

i like to use reserve archers as well; once my starting archers run out of ammo they can be replaced, which allows you to avoid having to sit there and get shot without being able to do much about it. particularly in defensive situations where you need to keep shooting throughout. if you field artillery, try to have reserve units to come on and replace them once they're out of range or ammo.

reserve cavalry of any kind is a good idea; coming fresh to the field they can coutnerattack strongly and see of the opposition, particularly fast light cavalry, which is not good for melee but great for pursuit. it's very annoying to see the enemy fleeing the field and realise that you have nothing that can catch them.

on the whole though, it all depends on who you're fighting and whether you're attacking or defending. i personally try and and start with the highest valour and overall best units i have, cos it's very hard for your reinforcements to come and win the battle for you if the main army's (and especially the general) is gone, so you might as well field your 'A team' from the off.