Rather simple actually. If your talking about an RPG world, then you will probably have FOG. If an object is in the fog (so you cant see if) simply hide the object. If the object is not in the camera FOV, then hide it.
If your talking about BSP type levels, they might be divided into a few different parts. If you ahve ever played the Counter Strike Training part, then you will notice whenever you get to a new part in the training simulation, it displays a loading screen for a few seconds, this is probably deleting the old BSP and loading in a new one. You can simply do this.
Cheers
RPGamer