If your'e actively trying to sell, here's a few points:
1.Declutter and depersonalise-People need to be able to see their stuff in a house
2.Neutral, but warm colour schemes throughout-get rid of the deep pink/purple/whatever colour scheme in little johhnny's bedroom-cream colours usually do the trick
3.Give every room in the house/apt. a purpose-put mirrors on the wall of a small room togive greater sense of space
4.If you have a garden,definitley tidy it up,put down plants in season, or failing that, a few heathers for year round colour
5.Rehouse the pet, and get rid of any doggy smells!
6.On the day of viewing,light a fire, and some vanilla scented candles will do-the smell of freshly baked bread(mmm..foccacia..!)does smack a little of overkill! also, keep all the lights on,esp during the winter months
Best of luck!
daithi