OK, here's mine. They always say first impressions are important so have a look at your front door. Does it need a lick of paint? How about going for a rich, impressive colour like navy blue or deep green and a tasteful knocker.
If your flat has access onto the street how about putting a couple of pots with evergreens like trimmed box or laurel on either side to give it a smart, well cared for impression.
Smell! I have been looking at property recently and I am always alerted to the smell first. I am a smoker (but I don't smoke in the house) but I am aware of the smell in another house which is not a nice one! I think fresh flowers and a quick sprinkle of shake & vac before a viewing! Also open all the windows and give the place a good airing but make sure you don't lose all your heat or they may think the place is cold.