Are you using beyond the sword expansion? It added an extra maintenance cost that you pay for cities on a different contient to your capital. This starts off reasonably low but once you have more than about 4 or 5 cities it really starts to get huge. However you can also now set up a colony on a different contients to overcome this. The cities liberated are given to a new friendly vassal civilization controlled by the AI and you no longer pay this type of maintenence for them. This can make it easier to get conquest/domination victory when you need to take multiple contients.
Other than that there are only the usual things to suggest to reduce maintenece. Make sure all your cities have courthouses and it helps to have a few cities with well developed towns. You can also increase gold yield with specialists, religious shrines, gold in trades from AI or even pillagaing if you can take the war weariness.
One of the best new ways to get big income is to found a coporation or two (preferably in you city with wall street) and spread this to the AI. The advantage here is that it brings in gold to help you economy, aswell as costing the AI more maintenence in their cities and hurting their economy.