i usually stand with my legs about 2 feet apart for stability.... (use the swaying method of focusing,,, it's the tried and tested method) i put my right pinky under the body of the camera, index finger on the shutter (obviously) and the other 2 holding the grip (I suppose this depends on the size of your hands tho,,, I use a 350D which is small, so it's real easy for me to find a nice grip on it)...... i rest my pinky on the palm of my left hand (near the wrist) and hold the lens with my left fingers and pull my left arm in close to my chest (making your left arm a kinda monopod ) don't tense your arm casue that will cause more shake than it solves..... relax,,,, move in out,,, when I get focus...FIRE...
i wouldn't recomend mirror lockup for handholding because you then have to press the shutter twice and on the 1st press it blacks out the viewfinder
As Brian also says, take LOTS of shots.... (my normal is about half a dozen per bug, one is bound to be in focus)
99.999999999999999 of my macro shots are handheld (like leo i can't be bothered with the tripod and i find that my monopod isn't versatile enough, it's easy enough to sway in and out but if you have to change the height of the pod by a few mm it's a different story, by the time you get the thing set up right your bug is long gone)
I just ordered a battery grip for it tho so i guess i'll have to modify my hand position (i really want the vertical shutter button tho and the extra battery life )
btw Brian , your cat is looking at you thinking (wtf is that crazy b***ard up to now, i wish he'd pay as much attention to me as he does to springtails )