One option is to use Buttons Window -> Editing -> Mesh Tools 1 -> X-axis mirror. It is a general option (if you set it on, it’s on even if you edit another object). This feature works even in asymmetric case unlike mirror modifier.
I recommend using mirror modifier whenever possible though. Sometimes you may find array modifier useful too. You can make radial mirrors with it (use object offset and empty). There is an example of that at http://wiki.blender.org/index.php/Manual/Array_Modifier .
Before we had modifiers mirroring was done by creating a linked instance of the object and then mirroring it. There may be some rare cases in which this method may be preferable. I am not sure though.