friendly fire

[alpaca]

Don't get too close though or it will alleviate the problem that your units are actually killing anything, too
The way to go usually is to either switch them off if you think you take too much damage through friendly fire, or to fire at units where the probability of hitting an enemy is much higher than that of hitting a friend.
Or get them around and shoot the enemy in the flanks (note that you risk a few archers when you do that, but it's often worth it).

[Caliburn]

Cheap archers can be used to break enemy cavalry charges (i.e. sacrifice them) so that the more expensive spear units survive longer and can wipe out the enemy, so sometimes it pays to place them right in front of the troops. Shooting the engaged enemy from the flanks is great as well, especially with gunpowder units or melee-capable archers (Aventurier, Venetian Archers, all those Turkish nasties and some of the spear-chucking fanatics, you name it).