Yes, indeed. Hide everything in your scene tab that is not in the camera frame, unless it is required to create shadows. Anything hidden in this way will not be sent to Luxrender and will reduce the memory requirement. Note, though, that although making surfaces null materials in Reality will save wasted sampling, the mesh is still exported, so it does not reduce memory requirements.

Although Reality does a good job in reducing such things when it converts materials, keep an eye on diffuse colour to make sure it is not pure white (255, 255, 255) as this is very wasteful, and unnecessary, and keep specularity very low, even on a fairly glossy surface. If you are using metals (which are fantastic) you might think about sacrificing some polish to avoid reflections on adjacent surfaces (say if it is jewelry next to skin). Typically with gold they look like red or orange speckles and never seem to clear up. You can also have this problem with non-metal texures with high specularity.

The cube trick really works. I use it by default, and although it cannot be used in wide open scenes (without postwork), I prefer tightly framed scenes in any case and use it for both indoor and outdoor scenes.