How to Macro, by Vattel

[Lintissa]

I like modifier macros too, here is the format I often use.

#showtooltip
/cast [modifier:shift] spell to use with the shift key down; spell to use normally

This can be used to save space on the action bar when you just have too many paladin blessings to cast (no longer an issue in Cata). Here the shift key allows me to switch between the normal and greater version of the blessing.

#showtooltip
/cast [modifier:shift] Blessing of Kings; Greater blessing of Kings

Or maybe as a rogue you alternate between two finishing moves.

#showtooltip
/cast [modifier:shift] Eviscerate; Slice and Dice