Last week, I helped Alex at Gizmodo test out the SmartDigital Handitheatre, an inflatable screen and projector for outdoor movie fun. Regardless of whether you want to $3500+ spend on the gear, there's one clear lesson for anyone aiming to set up an impromptu outdoor cinema: use a groundsheet.
One of the obvious issues with the inflatable screen was that the display area was covered in markings. That's doubtless partly due to other reviewers having it before us, but it's also a reflection, as Alex notes, of the fact that when the screen deflated, the screen collapses onto the ground, picking up dirt and twigs and all kind of gunk. Even if you're hanging a more basic screen from your backyard washing line, you could run into similar problems. Putting down a groundsheet is an easy way to minimise the problem.