The plastic flagging denotes different activities for the Forest Service and other forestry related organizations. Blue is used for partial cut areas, yellow for clearcuts, if you happen to be in or cross over an area laid out for logging. Other colors have to do with surveys and such. The idea is not to decorate the forest like christmastime with the stuff. A small piece (a few inches), not a huge long hank of it, placed at (for example) the point where you stop going one direction and change your course, is useful. A cairn would be less obtrusive, less obvious to all but those who are looking for it. Or look for landmarks that aren't going to go away easily and thus leave little trace of your travel.

SRS