Use the highest quality camera and acquisition format possible (i.e. ideally one of our 4K camcorders).
Pay special attention to the lighting. The background should be lit as evenly and flatly as possible. Strive for no shadows on the green screen.
Place your subject as far from the green/blue screen as possible, so your subject will not cast any shadows onto the backdrop. In addition, the backdrop will not create "spill" (i.e. reflected green/blue light) onto the subject, making keying more difficult during editing with any tool.
Make sure your subject isn't wearing clothing or accessories that are the same color as the background. Black is also not a good choice since it tends to absorb spill.
Smooth down any flyaway hair and avoid any fringe in the clothing. Avoid jewelry, like earrings and necklaces that might reflect blue or green or that might be difficult key due to the relative size of the objects.
Lock down the camera and do not move it during the shot.
Light your subject as you would like to see it in the final composite.