Use a beam scale, make a wooden base to make eye alignment (parallax error) comfortable. Check your loads as thrown regularly. Make sure the top of your weigh station is level.

Disregard Wonko's dangerous and idiotic advice.
Not to mention wrong on 100 levels of stupid.
Before all of you light up on me for STRONGLY Condemning Wonko's dangerous and idiotic advice, Read the link to Trapshooter's.com that Drew provided. Then come back and tell me how safe and effective poor reloading technique is.

Out there doing it best I can.