The best thing to fill stands with is lead shot; but it's expensive.
I filled the bottoms of my Target stands with 20lbs. of lead shot (20lbs. is not much shot, but it makes a huge difference IMO).
I filled the rest of the stand with playground sand.
I've read about people using kitty litter, I think that is ridiculous. You want to make the stands inert to vibration, and lead is pretty darn inert.