If you are on dirt, then tent stakes. Either with short lengths of thin rope or bungee cords wrapped around the center, or even better drill holes through the bottom of the legs and stake directly through the legs. You can stake down the cross bars on the stands legs if your stand allows for it, but since the stakes have to go in straight down it is less secure.
If on pavement the only choice is sandbags(or some other weight)
JAson