Most sofas are over-cushioned. The number that works for a standard three-seater is five: two matching at the outer corners, two contrasting inside them, and one lumbar or bolster in the middle that you can move when someone sits down. A two-seater takes three. A large sectional takes seven and stops there. Past that point you are not styling, you are storing bedding on furniture, and everyone who sits down has to relocate something first. The reason the odd number works is that it lets you break the symmetry at the centre without the ends looking accidental. Once you have the count, the interesting decisions are all about texture and size rather than about buying more. Two cushions in the same colour but different weaves will do more for a sofa than five in five different patterns, because the eye reads variation in surface as depth and variation in pattern as noise.

Cushion size is the thing most people get wrong. A 45 by 45 centimetre cushion, the default, is small on a deep modern sofa and disappears entirely into a soft one. Going up to 50 or 55 for the outer pair makes a sofa look furnished rather than decorated, and it costs the same. The second rule, which cushion makers including British Wholesales and Us Cushion both publish, is that inserts should be larger than covers by about five centimetres on a standard square, which is what gives a cushion the filled, slightly plump look instead of the flat-cornered one. Feather and down inserts do this naturally and can be reshaped; foam and polyester cannot, which is why an expensive cover on a cheap insert still looks wrong. If you are buying covers only, check the insert size the maker recommends and buy up. A lumbar cushion is the one exception to all of this: it is there to be useful, so it should be the size that supports your back.

Buy the cushions in pairs and the throw on its own. Start with two large outer cushions in a plain weave you would not tire of, add two smaller patterned ones, then a lumbar last once you know where you sit. One throw is enough for one sofa. Two throws on one sofa is a photograph, not a room. If you take one thing away, make it the size rule: going up one size on the outer cushions and buying inserts larger than the covers will do more for how a sofa looks than any amount of additional buying.