i use motorola walkies w/surveillance earpieces - that way we can communicate, even if not on set. i take off the earpiece plug (leaving just the stem) so i can still hear program audio in both ears with headphones on. simple, cheap, and effective.
my boom op uses a SD mm-1 to hear himself, and if he ever needs to hear the lavs we just feed a lectro IFB into it.
forgot to mention - these are my personal walkies, not production walkies. different freqs so nobody can listen in to our conversations!