As much as I like eating mushrooms, I dreaded having to clean them. It seemed to take ages and I tended to miss a lot. I somehow started using a clean kitchen sponge, the kind with the two sides. I moisten and squeeze until damp, and use the soft side on the mushrooms. It seems to make the unwanted particles disappear.
If I run into an unwanted blemish, I merely flip the sponge over and use the scrubber side. Needless to say, I now have mushrooms more often.
Just wanted to add that you can also take just the edge of a paring knife and peel the mushroom. You lose a bit, though not much, and if it's past the point of conveniently washing it, it's a good way to salvage them.