When using rolls of seamless background paper, the weight of the paper often causes them to continuously unroll. What is the best way to prevent this from happening?
If the crossbar is fixed and won't rotate (e.g. it has a pin though it), using an A-clamp to secure the paper to it will often work.
If the crossbar can rotate (or the A-clamp doesn't have enough grip or surface area to keep from turning itself) set the clamp so the handles brace against the upright support (or around the knob on a C-Stand). Make sure to do this on the back side so the handles are in contact with the upright support immediately.