My level of pre-battle planning usually depends on the type of battle. For siege assaults I usually create a specialist stack consisting of siege weaponry and strong melee infantry, usually a mix of swords, some polearms, some spears. When I attack in the field, I try to pack as much punch in the first line as possible. The aim here is to drive the enemy off the field and convince him to withdraw his reinforcements immediately, thus abandoning the field. With regards to that aim, my own reinforcements are negligible, since should the need for them really arise, I have failed to achieve my main goal: hitting the enemy as hard and as fast as possible and throw him off the field. That happens from time to time and the battle is usually costly then or even lost.
When defending, I aim for bloodshed. That's where reinforcements come into play and here I organize them too. First line of reinforcements usually are surplus archers/other missiles. Once the first set of archers has emptied their quivers, I call in the second set. Once their empty the battle has usually progressed to a point where the casualty ratio is significantly in my favour, I therefore have achieved my tactical goal. I might call in the next line of reinforcements, which is usually light cav to enhance my pursueing/capturing ability.