You can thin your wash, just don't over do it, too much water will increase the surface tension keeping the wash from setteling in the recesses. You can also give the mini a coat of gloss varnish before the wash which is supose to make it sink into the recesses better, keeping the raised areas cleaner, but I haven't tried this yet.
I'll usually wipe the wash away with my finger, which gives it a streaked or mottled look. If it gets too dark of grungy i'll just add another layer of the base color on the raised bits.