I agree with everything Edie and Pucca have said but here are a few of my pet hates/tips.
Pay attention to smells - I don't mean spray everything with air-freshener but if you know you have a smelly dog or a partner who smokes then try to air as much as possible. I detest going round houses that smell - I can easily envisage painting a magenta room white but find it much harder to look past stale fag smoke. I agree with Pucca about opening the windows (turn the central heating on at the same time if you need to). Steam-cleaning the carpets can really help persistent smells (and makes the carpets look better too).
Clean the bathroom, especially taps. Sparkly taps make a bathroom look much newer!
Open curtains fully to let as much light into the room as possible. If a room is dark then turn the light on before the viewing - that way you don't draw attention to the fact that it's dark, they may never notice.
People don't tend to notice what you don't have unless you draw attention to it - for example we have no storage at all - not even one cupboard. So our vacuum cleaner, brooms, mops and everything has to clutter up our kitchen. I put it all in the shed for the viewings - no-one asked me where I stored it or noticed that there was no cupboard space, whereas they would have noticed loads of clutter lying around iyswim.
Look up useful facts like the amount of council tax and rough fuel bills before hand.
People often worry about the roof and the boiler. If you have had either serviced or given a clean bill of health, or have any guarentees then mention it during the viewing.
Good luck!!