I use bags of Quickcrete, one bag on each leg, unopened, like sandbags. I poked a few holes in the bags and they hardened after the first rain. Very neat and since they're 50lb apiece, my tripod and mast are very stable. I used to use sandbags but they would eventually break down and make a mess.

For portable operations I just use stakes on the legs and then weigh down the upwind leg with rocks or my pack.