I think the most important thing is to de-clutter.
People want to think a house is roomy and low maintenance, so remove stuff from shelves and ledges and other "clutter" that fills up most of our homes.
Consider even renting storage somewhere and take some of your stuff there to make the house seem less cluttered and more spacious.
As has been said, a coat of magnolia on walls can make the room seem bigger and cleaner, but maybe do it yourself rather than get someone in.
Kerb appeal is also important and if someone drives up to your house and it looks a mess they may drive away without even coming in. So, as has been said, make the front look as nice as possible.
Have nice smells in each room, maybe fresh flowers around the house and so on.
You never know if you are going to get your money back, but it will certainly make the house sell quicker.
I remember the TV program House Doctor with Ann Maurice from a few years ago, and the advice above was the things she said in the program.
Here is her web site
http://www.housedoctor.co.uk/index.php