I've done the gravel trick, the mesh trick and the slate trick. For me, the slate has worked really well, the others not so much. I recommend finding slates or round river stones and laying the moss strand by strand on the upper surface until it's covered, then you can either tie it all down with fishing line or use a hairnet secured with fewer wraps of fishing line. I have ever had any luck with Java Moss adhering enough to form a uniform covering unless secured with non-biodegradeable line. Using slates is particularly easy because, in order to clean and trim, remove the slate, inverting it while still in the water, causing the moss to hang straight down (if you used biodegradeable thread, at this point it all drops off in a big sheet.) Then cut to a uniform height, swicsh around in a bucket of clean aquarium water, and then replace. Voila.
With mesh or gravel, you'll need to trim the moss in situ with a siphon running to take out the excess, and it's difficult to clean without sucking the substrate through the mesh or the moss right off the gravel.
That's my two cents. Reasonable minds may differ.