There was a great thread I can't seem to find right now that had a lot of great ideas. In lieu of that here are a series of house rules I've used over time (I don't use all of these at ones as some are mutually exclusive... based on the faction I sort of pick and choose)

- restricting armed forces upkeep to half of your trade income with the tax/farming/other half being used to pay salaries, build improvements, diplomacy, etc.
- regional recruiting - take half the income of the city (or some other combination like taxes + half trade, etc) to build units in that city. Build them on a 1:2:1 ratio or 1:3:1 ratio (peasant:regular:elite). When you field armies you draw them from these cities at your discretion. I really like this concept but it's hard to manage when you start taking losses. An alternate method is to simply limit military upkeep to half your trade income (civilized) or half your income period (barbarian or otherwise any faction that doesn't rely much on trade). some eastern factions do well with only mining income being used for upkeep as there are a ton of mines out there.
- max army size of 12 units (general included). the only exception to this rule is when storming a city - I normally just recruit mercs then disband whoever survived (though the way I treat mercs there are usually very few left at the end of a city assault heh)
- only allow one of each "royal unit" (greater than ~750mnai upkeep for infantry and greater than ~1000mnai upkeep for cavalry) throughout the empire. similarly, for elite units (greater than ~500mnai upkeep for infantry, greater than ~750mnai for cavalry) only permit 2 of each throughout the empire.
- unless facing horse archer factions, never more than 2 slingers/archers within the same army
- garrisons are exclusively made up of levy/peasant units. the only exception is when a field army is moved into a city specifically because it is under imminent attack or otherwise can not defeat the forces arrayed against it and needs to hide in a city.
- never retrain anything, always reinforce.
- always honor alliances - if you fail to honor it break it.
- whenever possible, attack the enemy army nearby rather than letting it come to you. this is based on my experience of the tactical AI being crappy on the attack but decent on the defence (mainly because it just sits its army in one spot and can't screw too much up).