"How can I ensure that I get the most accurate scale reading?"
Use a digital scale for best results. "Mechanical balance-beam scales [like those found in most gym locker rooms] lose accuracy if they aren't maintained and recalibrated on a regular basis," says James Montagnino, vice president of technology and innovation at Tanita Corporation, a scale manufacturer. "Since digital scales don't depend on any moving parts, they're less prone to wear and tear."
Place your digital scale on a hard, even surface; if your bathroom has plush carpeting, Montagnino recommends placing a flat board beneath the scale. "You should also make sure you're standing on the scale correctly," he advises. "Stand in the center, and be careful not to shift your weight to either foot." If your scale also measures body composition, don't weigh yourself first thing in the morning. "Fat contains less water than muscle, so the more dehydrated you are, the higher your body-fat reading," explains Montagnino.