I thought I read that it was best to breathe out rather than hold your breath.
I've found this to be best. Exhale in a slow controlled motion, it stabilizes your whole chest, neck, and head a lot better than just holding your breath. I am not sure but I want to say that is a technique military snipers use. I wouldn't know.
holding the camera steady is the easy part, it's your upper body swaying that kills the shot. I bought a monopod (slik makes an inexpensive one you can practice with called the Pro 600 or something) and I also tend to back myself up on something like a light pole or a wall. Also I don't know if this has been said, but your shutter should be at least as long as your focal length and in many cases longer until you get a good technique down. i've gotten lucky with some 300mm 1/8 handheld shots, but generally I need to be at 1:1 in that respect.