You can save rendering time by setting up render passes properly. For example, if you have a separate highlight pass and want to change the highlights, you only need to re-render that pass. You can then composite the new highlight pass with the other old passes.
For information on setting render passes, see Passes & Partitions.
Compositing is a practical way of optimizing rendering time. By rendering constant elements of a scene (such as the background, shadows, highlights, etc.) separately from the other elements and then compositing them together later, you can save a lot of rendering time. The number of passes you can composite is unlimited.
For more information about compositing, see About Softimage Illusion.
