For those who are fans of scrim, the most effective material that I have seen thus far is strips cut from the standard-issue individual camouflage screen. Used as scrim, the CADPAT camo screen material is lightweight, durable, does not retain water (and associated weight), it dries quickly, has a decent amount of "loft" to it (to break up the smooth outline of the helmet's profile), it is fade-resistant, and provides the right mix of CADPAT colours.
I've tried all of the other methods in the past. Natural scrim is a time-consuming PITA that soon wilts and frequently requires replacement to match your changing surroundings. When fresh and used in a static location such as the defence however, it is hands-down the most effective. Strips of T-shirt, CADPAT material, or Burlap/Hessian are OK, but depending on the material they tend to get heavy when wet. If you do use Burlap, make sure that you use the flame-retardent issued stuff that your CQ/QM can provide. Untreated Burlap is a significant fire hazard, and you won't blend in particularly well with your head on fire...
Avoid the "shroom head" or "afro" appearance that comes from applying too much scrim. All you will have done is created a bigger target. As others have said, the idea is simply to break-up/disrupt the smooth curved surface of your helmet - particularly when viewed in silhouette. This is based on the precept that there are no straight lines in nature.
Just my $.02. A creative CQ and/or QM should be able to arrange some extra CADPAT screens for use as scrim, particularly condemned ones that have been previously damaged. We used the CADPAT AR screens to make helmet scrim in Afghanistan, and it worked like a charm.
My $.02