I think a lot (probably too much) about force composition. I prefer to match enemy spears with my swords/axes, enemy cav with my spears, enemy ax/swords and archers with my cav. On defense, of course, I set up a spear wall with archers ahead (retreating behind the wall when the enemy gets close) and cutters (sword/ax) a few steps back on the wings, cav flankers and (if possible) missile cav out harassing the enemy. On offense, I position my force in loose groupings designed to deal with a particular type of threat -- spear and a cutter for most infantry and knights, one high-grade or two average cutters to kill spears, fast cav flanked forward for killing archers, heavier cav on high ground or flanked to assist units in trouble.

When badly outnumbered or outgunned, I take to the trees if available (except for horses, spears and archers). If not, I go for the highest ground I can reach (and if it has trees, so much the better). If neither is possible, I'll try to isolate one or more enemy units and kill them with my marginal troops. If it works, I'll repeat and follow up with the better units if it looks like I can win. If it doesn't, I'll set up a screen with whatever junk I have left and get the valuable units out of harm's way ("He who fights and runs away lives to fight another day").

In siege attacks (I've never yet had to defend a castle), I've started using as much artillery as I can. I buy mercenary mangonels whenever I can, and use them to breach interior walls first, then take out towers and other sources of defensive fire. I build my own catapults when I can, and use them for breaching outside walls (plus anything they can reach when the walls are down). Catapults are also really useful for river crossings - I set them up with some bows where they can cover bridge approaches.

Most mercenaries I regard as primary expendables. I'm paying them to fight and die, after all. So when its going to be a big battle, I'll buy a bunch of mercs to throw on the fire and weaken the enemy for my homeboys. This is especially true of sieges, which are always a fight to the death.