Here's a little gem I found that doesn't seem to have been unearthed in this forum yet.
http://www.sharecg.com/v/26853/Poser/MirrorMags32-
MirrorMags32 by les bentley, free on ShareCG!
A Python script that comes with 'poses' that are applyable from the pose library, works for both Mac and Windows, and is for Poser 5 or greater. WARNING TO MAC USERS: the documentation in the readme states dire consequences if you do not install it right, make sure you are following the proper installation proceedure when using!
What this does is has even numbered magnets mirror changes made to odd numbered magnets automatically! Seems to have more features as well, but my god, it makes magnets so much less hassle to work with. Just make your magnets, parent them to the appropriate body parts and then apply MirrorMag32 to the figure (applyable as a 'pose' in your pose folder). The magnets with a even number assigned to them (Magnet 2, magnet 4, magnet 6, etc) will dissappear, but will automatically mirror any movements made to the odd numbered magnets (magnet 1, magnet 3, magnet 5, etc).
VERY useful for achieving symmetry when using magnets to manipulate different side of a head, thighs, buttocks, etc!