PSE: at least with me, if I drink a lot of caffiene I notice that I tend to be in more of a hurry when I shoot. My attitude tends to be more: "just pull the trigger" rather than waiting for when the shot "feels" right. Again this is an observation about my shooting, it may or may not apply to you.

Also, I have noticed that when I am shooting with someone else, I never shoot my best. Its just too distracting. I don't mind shooting with other people, its a lot of fun. But in that situation, I expect to be more fun oriented rather than trying to shoot my best.

When sighting in, I like to go by myself. This way I can completely focus on the gun, what I"m doing, take notes, make adjustments, compare results, etc., etc.

As for the breathing, there are a number of techniques. The one you use should be what works best for you. However, as with much of shooting, consistency is key. What I do:
-acquire a close sight picture
-inhale (a normal breath) and hold
-acquire a final sight picture
-squeeze off the round
-exhale

I hope this is of some help to you.

~Robert