Clutter is your enemy!
Take a walk through your home and check it out as if you are looking to buy it. Does the house 'flow', can you make your way through the house easily, when you enter a room do you have any furniture blocking your entrance...
Look at the surfaces, is there chipped paint, any marks on the wall, nails without any pictures... Do you need to walk around a pile of shoes at the door / bottom of the stairs, does your coat rack take up half the width of the hall...
When we sold up our apartment last year, we did a walk around and realised that a lot of the rooms were looking 'tired', after 4 years without being painted they looked a bit worn... So we painted most of the rooms, painted the doors, installed radiator covers, changed handles on the doors and kitchen presses... We regrouted around the bath, and scrubbed the grouting to get it clean, it didn't look dirty until we had a cleaner bit to compare it with! Keep toiletries to the minimum, some posh soaps / bath stuff on show the rest hidden....
We rented out a storage space and dumped all the stuff that was around into it, clothes, suitcases, books (lots of books), some furniture, all our office stuff (lots of PCs), most of our CD / DVD collection...
Viewers open cupboards / wardrobes, if they do make sure that there is some space in them, they dont' want to feel that the house is short of storage!!!
Get all the windows cleaned weekly while your selling, natural lighting is your friend!
Re-arrange your furniture to give the best use of space... if moving the bed to under a window looks better do it, even if its colder for you to sleep there!
Check out the 'House Doctor' if you have satellite tv, it will give you a good idea of areas of concern.