As others have stated, unit maintenance is charged every turn, whether you are putting those units to good use or not. In addition to boosting revenue, you can consider disbanding units that you don't expect to need for the foreseeable future, including extra workers (can always be rebought for 310 gold when needed) or military units with no promotions (or few promotions). You will get a small amount of gold for scrapping the units and your maintenance will be lower.
Note that each civilian and military unit costs the same amount of maintenance, so there is no maintenance-cost reason to delay upgrading military units to more modern units.