I agree with what Kaorte said. I have one display tank (most of my animals are in racks) and I usually do a full clean out every 6-8 weeks. However, I spot clean when I see that there's been a mess made (it houses a corn snake so that's usually once a week).
As for what to cover, black construction paper, cardbaoard, or even a fancy backdrop work well. They can go on the outside of the tank. For my display tank, I have a forest scene that "blacks out" my corn's cage (not that she needs it...but it's for decoration). I have a BP in quarantine that's REALLY skittish so I have all 4 sides covered in cardboard to give him a little extra security. It seems to be doing the trick.
As for thermostats, if you can afford it, a Herpstat (even the cheapest Herpstat at $100) is well worth it! There's another model called Vivarium Electronics that's also pretty good. Those you can get from Reptile Basics and the cheapest one is $85. However, both those are considered some of the best thermostats you can buy. Although they are pricey, they are awesome! I have a Herpstat 2 and I love mine.