I was checking an office mate's swimming (can you believe I'm "giving lessons" - a bit pretentious, eh?) and I advised her to engage her core and she immediately looked smoother and had a better position in the water.

I breathe out through mouth and nose, as long as no water goes up my nose .
(Did I tell you gals I HATEHATEHATE TI drills? I get tons of water up my nose.

Our partner club is training by that method. I think once you have the stroke down, they are pointless because they break your rythm. I'm speaking of stuff like triple-zipper or triple-under).