I second the goby idea. Of course keeping your water parameters in line are crucial as well, however all sand seems to show some degree of build up over time.
My sand was starting to show some light colored film on it (it didn't look like cyano that I have seen, just some coloration) and I added a Diamond Watchman Goby about a month ago and he went to town and over the course of every other day seems to turn over every inch of my tank. The sand looks great and he makes some really cool burrows.
At this point he is my favorite tank critter to watch. Don't add one unless your sand is very fine as they pass it through their gills to filter out critters and micro stuff. I think a healthy refugeium is good for filtering critters as well which I have. I do try to target feed him a few times a week with whatever I am feeding the tank to make sure he is happy as well.
Good luck,
Chris
