Best thing Ive found to do, is simply walking a double grid until I find as many as I can. I know how many I fired, and try to recover that. Knowing the count helps a lot too.
Most of the time, I usually do get most of it. Sometimes I come home with more, sometimes less. Tends to even out too.
If youre standing still, and there is no breeze, a tarp might work. As soon as you start moving around as you shoot, even just a little bit, they are pretty much useless. If a breeze pops up, even just a little one, forget it. And youre looking for any brass that was on there, "again".
I have a couple of the Cadwell brass catchers for a few of my AR's, and they work great. You really don't even notice them on the gun as you shoot, and Ive yet to have a stoppage because of it being there, even when running and gunning. Just remember to close the zipper on the bottom before you start shooting. Ask me how I know. Wondered what was hitting my legs and feet as I was shooting.
Most of my guns will throw things in a general area, so you know pretty much where to look. The only ones that don't, are my 1911's and GI Carbines. They just chuck brass everywhere, with no rhyme or reason.
AK's chuck them out at around 1-2 o'clock, and a bit farther out there. HK's just chuck them about half way into the next county. Its almost like they have rocket assist.