My tactics vary depending on the enemy army's composition...

Often I use the static line:
SSSSSSSS
AAAAAAAA
C_______C
___RRR___

Where S are spears, A are archers (Or crossbows or muskets), C are cavalry, and R are artillery...

This is when I don't intend to do anything fancy.

Sometimes I'll put the missile troops out in front on skirmish, but only if I want to provoke an early charge and generally only if I have high quality missile troops who can do ok in melee.

Two somewhat unique situations I've run into lately...

Playing as France I had 2 units of Armored Sergeants, two units of either Crossbows or Aventurier, and my general's bodyguard. I was significantly outnumbered, but the enemy was mostly town militia so I wasn't too worried. The general's unit was Armored Sergeants though, and they had one unit of peasant archers.

I deployed in the static line formation expecting them to hit me and try to overwhelm me with numbers, but instead he had deployed somewhat back up the hill in order to force me to approach uphill where his peasant archers would be able to outrange my high end crossbows...

I didn't feel like marching up the hill... and also all I could see was his general's Armored Sergeants and the Peasant Archers out in front of them. So, I briefly considered trying to flank with my general's unit, but I'd been having REALLY horrible luck with generals dying in successful charges. So I thought about it and came up with an interesting solution.

I split my infantry into two groups, one spear unit and one crossbow unit each, and had them move into position on either side of the enemy, keeping my general's cavalry in the middle. The enemy couldn't afford to turn to engage either unit of infantry without being shot in the back by crossbows and having my general's unit charge them in the side, so... They just sat there while my crossbows wiped them out. Naturally I focused on the general's unit since the Peasant Archers couldn't actually hit my troops now that they were level with them.

They sat there until the general died and his unit was nearly wiped out and then they routed. I charged in with my general, and the Archers routed as well, seeing the general's squad leave and a unit of heavy cav closing on them. I pursued them taking tons of prisoners until I got close to the woods and noticed his town militia were hiding in there... peeled off, and sent the armored sergeants in to flush them out.

I think I took all of 5 casualties in that battle.

Another trick... I was playing as Venice, moving against Milan who had my city of Florence besieged. I had 8 Pavise Crossbow Militia, 10 Italian Spear Militia, and 2 Cavalry Militia. He was bringing a lot of horse archers and other cavalry to bear (Not sure where he got them). I knew I could beat him if I could keep his men off my missile troops long enough to break them.

So I thought about it and came up with this idea:

_M_M_M_M_

S-S-S-S-S-S
CAAAAAAAAC

Basically I deployed four of the spear units in a very compact line, probably 8-10 ranks deep each, and then flipped them to Shiltrom formation. They're represented by the M's.

I left a fair sized gap behind them and then spread the other spear units in a long thin single line there, and backed them with my Pavise Crossbow Militia, with a unit of Cav Militia at each flank.

Effectively, it created a forward barrier the enemy had to either charge, which would obviously beat up their cav quite badly, considering they were in Shiltrom, or else they'd have to bypass them, charge the spear behind them, and risk the squads in Schiltrom dropping formation and hitting them from the rear while they were engaged.

I basically annihilated the AI in that battle... we were fairly evenly matched, but I came out with 1700+ kills to his not quite 300.