The agents Cluttons Daniel Smith even send a staff member round to clients' homes before a viewer is due, to make sure blinds have been lifted and curtains drawn, to maximise light in the rooms and create the right impression.
Cluttons also advises removing net curtains altogether (they make a room look as though its eyes are closed), having windows cleaned and investing in curtain tie-backs to make the room look more dressy. It may sound nit- picky, but the company's marketing policy is paying off. Good first impressions, it claims, have led to more viewings and sales.
Sales have been known to fall through because a buyer insisted on having the curtains included in the price of a property. "It all gets out of perspective," says Noel Flint of agents Knight Frank.
"Sometimes the curtains were made to match the bedspread, so the vendor wants to take them. We've even had a buyer demand the bedspread as well. But some people get very attached to their curtains, especially bachelors who consider they paid a fortune for them years ago and don't see why they should buy new ones now."
Well-made curtains which link with the walls and furnishings set off the room, says Noel Flint. They also prevent the echo in an empty room which can make it sound unfriendly.
"But curtains in strong colours or heavy material can seem dated, and give the impression of the house being caught in a time-warp. And if the curtains are dated, what does that say about the wiring, or the plumbing?"
Buying new, ready-made curtains - easily alterable, so they can move with you - may be easier. Among the simplest and best value are those from Off The Rails, from the Futon Company. Unusually, they sell one-size, single curtains, so you're not stuck with a conventional pair if you want to mix colours, or cover a three-curtain window width.
At pounds 29.95 for a hand-woven curtain, you can afford to cut off the length for smaller windows. They have generous button-up loop tabs at the top to slide onto any curtain rail, so no fiddly curtain hooks are needed. Yarn dyed, they are 100 per cent pure cotton and come in nine colours, including a bold burnt orange, a rich raspberry, and putty. Each curtain is 230cm long by 150cm wide.
Neutral-coloured cotton/ linen mix curtains also cost pounds 29.95 each, but coloured muslin curtains are just pounds 8.95 each. The heavier, but fashionable one-size velvet curtains (230cm by 110cm) cost pounds 34.95 each and can be used as room dividers, or around four-poster beds, as well as over windows. Colours include terracotta, lilac (this year's must-have shade), electric blue and peach. They can be bought by mail or from the stores.
Also good value, The Pier's tab-top single panel in sheer white with bright spots (100cm x 210cm) costs pounds 29.95, and the metal curtain rod to go with it costs from pounds 6.95, depending on length.
The Pukka Palace also sells loop-tab-top single curtains in dyed cotton voile, in star, leaf and swirl designs. Chunky rope tie-backs cost pounds 9.50 each. As a clever variation on a theme, the pale grey patterned curtain fades from top to bottom, as does the golden yellow, while the indigo fades from sides to centre. They cost pounds 18.50 each and measure 110cm by 228cm. At that price, you could even afford to leave them for an admiring buyer.
Marks & Spencer's ready-made Ottoman curtains in unfussy styles are unlikely to date quickly. They come with loop top, pelmet top, pencil pleat or sail-style eyelet top in cream, dark indigo, terracotta and pale yellow. Prices start at pounds 65 a pair.
And if you do want your curtains to match the duvet cover, Coloroll do a range of ready-made curtains with matching bed linen. Perfect for spring, the Chantilly curtains, in lemon and white with spring flowers, cost from pounds 45 a pair.
Off The Rails 0171-7369190; The Pukka Palace 0345 666660; The Pier 0171- 814 5020; Marks & Spencer Home Direct 0345 902902; Coloroll 0800 0564878.