very early game, i'll put at least 2 archers and 1 warrior.
as soon as i can get axemen, i'll try to put 2 in every city.
if i know the AI has mounted units, i will add spearmen with the medic upgrade.
i'll usually convert my archers to longbowmen, but that depends on unit exp.
by the time macemen arrive, i'll work on converting all of my axemen (again depending on unit exp.). i would rather keep an axeman with 17exp than upgrade him. it rolls back to 10exp when converting.
by this time, i will usually have 2 or 3 stacks that roam around. one stack will be mostly mobile units (horse archers, knights, cavalry, etc.). the other will be high exp units. i'll also include catapults or trebuchets in those stacks.
from then on, i'll add units to border cities. with gunpowder, i'll usually skip musketmen and wait for rifling to get cavalry and riflemen.
one problem i'll run into, more so than dealing with AI attacking, is that i'll run low on money. when i'm running vassalage and theocracy i'll build a majority of my forces. so depending on the AI relations, wars, funding, and tech. research, i'll build a considerable force that can be easily updated in the latter eras.
by industrialism, i'll usually have 6 units per city with at least one medic in the stack. and from there, i create roaming stacks (or assault stacks).
one thing to consider, however, is that i play on standard or smaller sized maps and on marathon. i can't really say how effective this would be on larger maps or different speeds.