The basic principles to be concerned with when trying to decrease render times. These are the main things to look at, in order of importance:

1. Polygon Count
2. Construction techniques
3. Reflection and Transparency
4. Material Choices
5. Render Settings

The first thing to do is make sure you are not needlessly adding geometry that increases calculations. Next, is to turn on and off materials to see if a particular material is increasing render times. Also, optimize your reflected materials for performance by excluding objects or using Environment maps instead of true reflections.  Blurred Reflections are very slow, as are Area Shadows.

Finally, you can use the Compositing Tag to selectively set objects to be hidden from calculation intensive operations like Global Illumination, Reflection, Refraction.