If you want to be really clever, find a couple of colours to work with - a rich saturated shade and a very pale neutral colour - and paint the skirting, ceiling, cornice and door frames in the pale colour. Use the same colour to paint a band of about three or four inches, vertically, at the corner of each wall and horizontally below the cornice and above the skirting. Then inside that square or rectangle use the very rich colour.
What you're doing is creating a big coloured panel on the wall which makes the corners of the room almost seem to dissolve. It tricks your eye not to see the perimeters of the room and works marvellously.
A light floor with a very strongly coloured rug in the middle of it has the same effect - you're pushing the edges of the room away from the centre.
Also, concentrate as much of your furniture into the middle of the room as you can. Putting it all on the outside of the room leaving a lot of space in the middle, draws attention to the perimeters, exactly what you're trying to avoid. The British are really naughty about this - they're forever sticking their three-piece suites right up against the wall so it's just like a doctor's waiting room.
Laurence Llewelyn-Bowen has just launched a range of wallpapers and sheer curtains - available from Homebase, B&Q, Homestyle, Focus Do It All, Great Mills and independent decorating shops nationwide. For information/stockists, call Freephone 0800 3288452