then you really need to get into the role a bit more

for starters, try capturing a far off city with a huge population, then exterminate them and loot the 10-20kdenarii from their burning houses. now say these were the Carthaginians. now find an Ally of crathage and send your diplomat to give the captured region to them. hmm now carthage have a problem, they want thier capital back but thier weaker a\lly is holding it with no garrison. oh well, war it is then. mwwahaha. your army slinks off into the moutains to find another target.
another enjoayble one for me is plague distributing. when you get a plagued city build some spys and bung them in a fort in the countryside somewhere. you now have agents capable of not only spying on cities but giving them all plague. oh yes...

. when they get old put a new young spy in the fort to keep the weapon available.
dont forget that you can also speed up the siege,fight,repeat boredom with spys opening the gates for you (just dont use sick ones)
training and developing master assassins is a must.
take an army, the best youve got, and find a perfect battle field to defend in someone else land. if you have a huge army that cannot be beaten because of a tactical possition then you can essentially keep it there forever topping up with mercenaries. Once you have such a poblem lodged firmly in the huge Roman heart, they will concentrate all efforts to remove you, leaving thier borders weak and your settlements un-hindered.
Work deeply into the politics of the other factions. i once paid the gauls a regular tribute as an offer that eventually mounted up to 350kdenarii over 150 years. sure helped me kick the romans out of europe though. Eventually they turned on me and i had a weak barbarian faction to defeat rather than a mighty roman one.
learn to defend your regions on the borders, with forts and scout armies. dont ever let them into your lands and you dont have to fight the boring seiges every turn.
hmm...thinking of more...when playing as a weak faction, send your spies out to uber developed top factions and look to which settlements have a foundry to produce silver standard armour and weapons. take a ship and an army and nick it off them. capturing people places and destroying all thier building, then leaving a tiny population on full tax with no garrison hugely sets back your opponents.
man, i love this game
R: total realism patch is by far the best.