Tripod first choice, monopod second, brace yourself against something solid third, lastly breath control, etc. Use a shutter release and/or mirror lock-up if you can.
Seriously - StMarc's suggestion is very good - but only as a last resort. An external, solid support will always be better. I recall a column from Herb Keppler (?) in the 70's, probably Modern Photography, maybe Popular Photography, on how to get the best steadiness. I'm more or less parroting his suggestions, which follow my own experience.
If you must, you can increase your steadiness somewhat by "internal bracing." Use that nice, wide camera strap as a loop around your shoulder and tighten it by flexing your arms. Not so much as to cause muscle tremors, but the additional firmness will buy you a little more steadiness. I usually take a deep, slow breath, exhale partially and shoot. Don't get so excited that you jerk when you push the shutter. Smoothly does it. Yeah, you know this - but it's personally one of my biggest problems.
You'll get generally steadier results with wide-angle than telephoto.
DISCLAIMER - As far as I know, I've never been near a lens that focuses while it shoots, so these comments are based on my nearly forty years of pretty much exclusively manual 35 mm work.
Keppler found he was pretty good down to about 1/8 second, handheld, using - as I recall - a normal lens.
Okay - an appropriate photograph - hand held, one second (I think) exposure, holding camera on top of a concrete post. Not great, but a personal favorite for sentimental reasons.
Remember, the model won't be able to be steady for more than a fraction of a second, either. Unless, of course, the model is one of those magnificent little warriors your cousin creates.