When to enabled "Index" on a field in a table?

Is there any rough guide or rule of thumb for when it is helpful to enable the "index" for a field in a table?  My database includes a lot of calculations with select statements, and I'm hoping that judicious use of the index option might help improve performance. But I'm also aware that overuse could hinder performance.  For example, is it best just to Index fields I know are being used as part of select statements?