The balances must be clean and have readings removed and the doors closed when you have finished a weighing. Hot materials should not be weighed on the balance. Reliable weights cannot be achieved. Do not weigh anything that has solvent on the outside of the container. Wet containers lose water rapidly and will not give accurate weights.
Water also corrodes the balance pan.
Do not weigh any solid by transferring the sample to weighing paper on the balance pan. Weigh by difference and do all of your sample transfers outside the balance. The balance doors must be closed to obtain reliable results. You should use the same balance for replicate and differential weighings in an experiment. You may use different balances for different experiments.