Android uses RAM similarly to OS X and Ubuntu (Linux). It's actually it's goal to use up as much RAM as it can, because it will execute those processes faster from RAM than from an SD card (or internal storage w/e). And if you need RAM for another process Android will release all needed RAM for that process to run smoothly. There are a few apps that are "stupid" and don't empty the RAm them selves but ask the user to empty it for them but they are few as I said.
Why would you want your RAM to stay unused anyway?
Task killers can clear up your RAM but this way they will slow your phone down. You can check out which apps use up the most RAM and uninstall them, like Google Maps, Facebook, twtr apps, some games run in the bg from boot, too, like Front line commando etc.