Some say muscle injuries will heal with fire / Some say with ice

From what I've asked of different people (physical therapists, yoga instructors) for my shoulder injury (trapezius/rotator cuff), I've heard conflicting recommendations. Which is the way to go -- heat, or ice?

IIRC from my PT/Athletic training days, the only hard and fast rule is that you have to ice within the first 48-72 hrs if it's an acute injury.

This doesn't seem to be that, though, so ultimately it is kind of an empiric thing, whatever works better for you. I prefer ice myself, but heat is generally a more pleasant experience (unless you're doing it wrong & you burn yourself). If you need to move the joint right afterwards, though, put heat on it, since they'll loosen the joint up a little temporarily, whereas the cold treatments reduce the local blood supply.

I've iced so much of myself that the sensation itself doesn't bother me much anymore, though I have noticed lately that mornings after I ice my shoulder (I ice before I go to sleep), my shoulder feels particularly achy. I guess I'll break out the heating pad again, though.

i've heard that you should't ice a strained muscle for 20 mins at a time

more than 20 mins at a time

This is true. Ice works by constricting the local blood vessels, but after like 30 minutes (give or take) the vessels will begin to open up again, and so it's not really effective anymore.