This script allows you to work on a low res mesh while seeing a true representation of the mirrored end result underneath. The mirrored result is complete with welded seams which are necessary in order for the Smooth Mesh Preview to be correctly smoothed across the centerline. Improving on Maya's native "reflection" transform ability, this script will also allow you to add new nodes like 'Extrude' and 'Insert Edge Loop', for example, and they will be mirrored as well.
Installation: Copy and paste the MEL script to your local "\Documents\maya\[current version]\scripts\" folder. Restart Maya and type "realSymmetry" from the MEL command line. Or create a custom shelf button with "realSymmetry;" as the command.
Note: When you're done modeling and want to end the Real Symmetry setup, select your new reference model in the Outliner (since you currently can't select it in the viewport) and access its Shape node in the Attribute Editor. Find the "Object Display" --> "Drawing Overrides" section and either disable the "Enable Overrides" option or change the "Display Type" from "Reference" to "Normal". Now you can delete its history to unlink it from the low res cage mesh and then select and delete the original low res cage mesh, if you want to.