Bribery is your friend.
often times you can bribe a full banner army for 1000-4500
depending on it's leader and composition i think, and this compared to unit costs is cheaper then keeping an army near a border settlement that has upkeep to pay every turn and cost between 300-500 denari to recruit per unit plus prolly has an anual upkeep of at least 200 a piece. of course sometimes you are attacked by the faction heir or leader, and they cannot be bribed. and that is this strategies weakness. but in general it works quite well.
also, there is the "offensive diplomat". i was carthage in corduba, and fighting a war against spain and gaul at the same time. very bad for me as i was also engaged in a hefty battle with the scippi and bruti on lower italy. so i didn't have the resources to keep a big 2 banner army there to defend from both spain and gaul(who were by now sending armies together.)
so, i thought of something interesting, what if instead of leaving my diplomat in my own settelement , i took him out to the enemies. genius. i found gaul had a huge army built, with no faction family yet in it. so i slippped in a bribe, paid 4000, and presto. gaul was out of comission for about 5-6 turns. i then applied it to all factions in which i was actively at war. when i felt they had a big threatening army i used a diplomat(whom was allready there) to simply pay them to disband. it worked wonders. i have also found it works against little armies who would normally slaughter your peasnts, but can be paid off fopr 400-1000 denari.
what does everyone think of this strategy?
ps. sorry for long post