Architectural Illustration of:

Rendering Rule #19

Reflections. I find it's useful to add reflection to most objects in an architectural rendering. You'll need to adjust this based on render times, but if you look around you'll see that most objects have some level of reflectivity. The next trick is to figure out how much reflection to show. Often I like to crank it up to the point where it's obviously too much, then dial it back down.