How about some tips on scanning?

I honestly thought that the photos that I scanned andstored on my external HD would appear better than they have been. If there is much dark (like a black background on a picture shot at night), it seems like there is light colored dust floating around in the scanner and putting white spots on the dark background. If I scan the same picture more than once, the spots are in different places each time. The picture and the glass on the scanner are both clean and lint free, outside.
Finn