Consonant Phonics is a PowerPoint presentation that shows how to teach the sounds for M and N through drawing. The "m" and "n" are incorporated into the drawings, making connections between the physical act of saying the sound, the visual representation of the letter/picture, and spatial act of the students drawing the pictures, etc. I've always used this method to teach the difference between the two letter sounds, and I've never had to teach the same students more than once because they always remember it.
Contrasting B/P, F/V, and S/Z are also included. More will be added (and updated for free) as I have time to do so.