I had to seal up a tent myself a couple of years ago and used masking tape (cant recall if I put it inside and out or just out). It worked great and is still on there and kept the dust to almost zilch. The sheet over stuff works pretty darned well too unless you get the UBER-fill-your-tent-with-play...

I have a tent that has nice large mesh vents along the ceiling. Nice for keeping cool and perfect for letting the dust settle in. I used masking tape to completely seal the vents up. I applied it at home, packed the tent back into the bag and it was still in place when I set it up on the playa. Cons...