Another thing to think about is your breathing. If you’re holding the camera, using a shoulder rig or not, your breathing might affect what you are shooting. So you have to practice not breathing with your shoulders, and breathing from your diaphragm. This will help you steady the shot.
For an example of my work, check out our segment “Geo with Joe.” For the entire series, I had a DSLR on a tripod head that I held while riding in the passenger seat. All I did was rest the “pan arm” on my shoulder and then held the rest using both hands. We hit some bumpy roads along the way, but for my first time doing car shoots, I was impressed with myself.