OK so two questions; First the sand bed exchange.
Great idea, I would never, ever do it. With all the strange pests going around, I would never risk bringing in someone else's sand and infecting my tank. No offense to anyone, but we really have no 100%, for sure safety from pests and I personally, would rather be safe than sorry.
That being said, the sand could always be put into a community "tank" and watching for several weeks like a QT. That's the only way I think a sand bed exchange would work.
Question 2, how do you take care of a sand bed...
You should be looking at both your live rock and live sand as animals in your tank. These animals need flow to expel waste and eat, they need light for some beneficial organisms to grow on them, and they often need food. In the case of our sand beds, they have tiny plankton consuming organisms such as copepods, amphipods, etc. This isn't usually where sand bed maintenance becomes a problem but remember that all those tiny bugs and worms are continually turning the sand over (keeping waste at the top).
The second way to help your sand is on more of a macro scale. If your tank has high phosphate and nutrients, guess where all of that is going to settle. Strong flow will keep these nutrients suspended for long enough that they can be taken out via filter feeders, protein skimming, or mechanical filtration. The other step is to employ plenty of creatures to keep the sand clean. I recommend 1 fighting conch for every 100 gallons or so, Pacific nassarius snails (LARGE variety), and detrivore starfish. My favorite stars are the black or red brittle stars. They stay fairly small and are a true detrivore unlike their carnivore green cousin.
Sorry for my book of a post,
John