What is the proper way to breathe while trying to achieve an OBE? I always breathe in through my nose and out through my nose. I find that it awkward having my mouth open and trying to breathe out through my mouth while trying to relax.
There is no right or wrong way to breath, imo. Breath the way which is comfortable to you.
From my own experiences, I'd suggest you breathe through your mouth. Have your jaws and lips very slightly apart.
I think too little a fuss is made of breathing, jaw and lip settings during the OBE process (no doubt many will have contrary experiences).
In one of Robert Monroe's books, he mentions (rather insignificantly) that the jaw seemed to act as a "tuning" apparatus or mechanism for the vibrations. In my opinion, this tuning apparatus is very important for the conductivity of the vibrations.
So, I'd say: mouth slightly open, breathe through your mouth (and place your left hand in contact with your stomache area or left leg--I suggest this because nearly all of my OBEs were preceeded with these criteria, though, undoubtedly others might suggest to you that it doesn't matter).
Mind you: While I've had many OBEs, they've really pretty much only been spontaneous, occuring after regaining consciousness at some point during naps or a night's sleep. And, as for the rate of my breathing, it's usually been quick once the vibrations have been strong enough to project.
Mind you: I'm just a lurker on this forum and have no mana here.