To put it simply, sandbags are a great way to keep your light stands from falling over. Typically a canvas bag filled with sand (I know, weird!), they also have a handle attached to make lugging them around easier. You can also use them to keep your PVC dolly track from rolling around or as a low-angle place to put your camera. You can get them for about $6 on eBay, as long as you are willing to buy four at a time. Finding the weight to put in them (sand, rice, beans) is your responsibility.
There are some DIY plans out there, but they all require sewing skills and none of them seemed to best the eBay $6 version. I liked the version on DIY Photography which look great and have a more vertical shape (and great color), but you still have to sew them. That one is also filled with bird shot (small steel pellets used in shotgun ammo) which adds even more cost. There is also a very cheap duct tape and plastic bag version, but I was looking for something a little more polished.
After poking around in some online forums, I came across a post about ankle/wrist weights. The poster commented that he used these exercise tools that he picked up in thrift stores. I liked that idea, especially since they came with their own weight and I would not have to add cost or time by buying rice or finding sand. It looked like the perfect solution if I could find them at a price better than $6.
And I did. Wal-mart carries a pair of Gold's Gym branded 10lb. ankle weights for $9. You get two weights whose shape will conform to whatever you place them on. They have 5 segments filled with sealed sand packets you can remove to lighten the load. The included velcro straps make it easy to place around non-flat surfaces without falling off. They are black in color, which means they will disappear when placed on any set (which may be a disadvantage when you go to pack them up later). Some day-glow tape would make them stand out more if that's what you're going for.
I tested them out on a PVC light stand and am very pleased. I first dropped them on the flat base and the weight kept it still. Next, I added the third leg and wrapped the weight around it with the velcro straps. This configuration worked even better and gave a lot more stability to the post, allowing it to do much more (like safely holding a round paper lantern). I think these Wal-mart weights are a success!
I like repurposing things even better than the DIY route. It usually takes no effort, the item comes from a factory so it looks great, and if you don't like whatever it is you're trying to get it to do, you can still use it for the original intent. In fact, when these weights are not performing on set, I can still wrap them around my ankles and go running. I like that.