In my backcountry kit I carry a small signal panel like this one:
Battle Systems MPIL
As I was going through my kit recently, I thought a good addition might be a 10' piece of pink flagging tape.
It weighs almost nothing, and takes up almost no room. In high wind, it would flap in the breeze pretty well, and could possibly be waved around on the end of a stick/trekking pole to give a searcher a better target to spot. In addition, I think it could be tied onto a weighted line for a daytime "lasso"- similar to what .mil folks might do with an IR glow stick at night.
If there's anyone here with any aerial search experience, I'd really appreciate any input to see if there's any validity to my thinking. Regardless, it's so light and compact I'll probably leave it in.
Aside from rescue uses, it could be used to mark trails or turnoffs, other points of interest, or used to denote location if you've got a group coming down the trail behind you that you'd like to link up with.