Most of the rules i play by are here in this thread already in one post or another.

One I've come to love is not using full stacks, but instead half stacks. The only factions where this isn't feasible are:

Hayasdan - Because you need every unit you can get for the first 30-40-50 years of campaign.
Pahlava and the steppe factions - Because 10 units of HA isn't enough arrows to defeat a full stack of anything.
Rome - Because of the way a legion is laid out. 10 units isn't enough to form a triplex acies.

But every other faction, 10 units is enough to give you a front line of heavy/medium troops, some light supporting troops, 1 unit of missiles, 1 unit of cavalry and a general, and that is more than enough to defeat the stupid RTW AI, but not too much that you are always guaranteed a win.

I'd go a step further even and reduce that number to 8 units, if you were playing as, for example, Epeiros, and went to war with Rome, or even better... Gauls or Germans or Dacians.

Barbarian factions had a habit of hugely outnumbering their enemies on the battlefield... only using 8 units against them when playing as a phalanx faction would represent that quite nicely.