It's an oft-repeated claim that two-thirds of women wear the wrong bra size, and while that research isn't iron-clad (it was based on a self-reporting questionnaire), an uncomfortable or ill-fitting bra shouldn't be ignored.

"Many women guess their bra size and some stick with the size they've always had, even though their body may have changed shape over the years following pregnancy, losing or gaining weight, or simply because they've got older," says Karen Stottrup-Thomsen, assistant lingerie buyer at John Lewis.

How can you tell if your underwear isn't optimal? Kelly Dunmore, chief lingerie stylist at Rigby & Peller, recommends using the 80:10:10 rule.

"80% of the support comes from the band - so you need to make sure it is firm but not tight, 10% comes from the straps, which need to be secure but not suffocating, and 10% from the cups, which need to be secure against your body."

Whether it's too loose or digging in, if your bra doesn't measure up, it's time to take action. Follow our guide to get your best bra fit ever...

SIZE MATTERS

Online bra shopping offers a world wide web's worth of lingerie brands and often a larger size range, so it's good for very large or small busts, but you'll need to get handy with a tape measure before you start clicking.

Begin by breathing out and measuring horizontally underneath your bust, and making a note of the measurement in inches. This is your band size.

Next, measure around the largest part of your chest, but don't pull too tight. This is the bust size.

Head to www.debenhams.com/lingerie, where there's a step-by-step measuring guide and comprehensive table for working out your bra size.

However, you shouldn't automatically start shopping for that size alone.

"The most common fault is wearing a bra that has been too loose on the under-band, therefore your new band size will feel tighter," says Zoe Norman, lingerie buyer at Figleaves.com. "Try different shapes to see what suits you and what feels comfortable."

Figleaves.com also has a useful trouble-shooting tool that uses your current bra size as a starting point to identify the cause of any niggles and suggest a different size that will correct the problem.

EXPERT FIT

If finding the right fit at home is proving difficult, seek out an expert at a specialist lingerie shop or department store.

"The advantage an expert fitter brings is that not only will they fit customers into the right size, but they will ensure the style is suited to bust shape," explains Karen Stottrup-Thomsen, assistant lingerie buyer at John Lewis, where free fittings are available without an appointment. "The fitter will bring a number of bras for the customer to try on and will work from there to get the perfect fit. They can offer advice on whether the bra is a suitable fit, while ensuring straps are correctly adjusted, and so on."

She advises wearing your favourite bra to get measured, and talking through with the fitter any concerns you might have: "It gives the customer a chance to learn more about what works for them as an individual, and the fitter can introduce new styles or brands to customers that they may not have thought to look at."

GO BESPOKE

For the ultimate in bra-fitting perfection, Rigby & Peller offers a 'Made to Measure' service, which involves two fittings and results in a one-of-a-kind piece tailored precisely to your body.

"Naturally, the fit will be better as the lingerie will be made to your exact measurements, ensuring in-between sizes, such as 31-inch backs, can be truly catered for, or if you need one cup bigger than the other, for example," explains Dunmore.

Bras start from £305 while repeat items cost £260, as your pattern will be stored in the atelier - alongside those of the brand's celebrity and royal clientele.

BRA SHOPPING

:: Marks & Spencer Mimi Floral Underwired Push-Up Balcony Plunge Bra (A-E), £14, and matching Low Rise Bikini Knickers, £6 (www.marksandspencer.com)

:: Marie Meili Angela T-shirt Bra, £24, and String Brief, £10 (www.asos.com; available September 1)

:: Maison Lejaby Gaby Bra, £39, and Brief, £32 (www.maisonlejaby.com)

:: Myah Full Support Balconette Bra, £32 (www.bouxavenue.com)

:: Silent Assembly Vita Contour Plunge Bra, £49 (www.silentassembly.co.uk; available October)

:: Collection by John Lewis Phoebe Balcony Bra in Smoke Blue, £25 (www.johnlewis.com)

:: Simone Perele Caresse 3D Spacer Bra in Pearle, £61.95 (www.rigbyandpeller.co.uk; available in store only)

:: Freya Deco Vibe Underwired Moulded Plunge Bra, £36 (Figleaves.com)

GET THE LOOK

The Saturdays singer Una Healy added a bright touch to a chic but casual ensemble - and dispelled the myth that redheads can't wear red - when she jetted in from Dublin recently, carrying a Cath Kidston Button Spot Medium Leather Tote, £60 (www.cathkidston.com).

BUY IT NOW

An ample bosom can be both a blessing and a curse, and can make finding clothes that fit correctly on your top and bottom halves much trickier. That's where new brand Pepperberry, available at Bravissimo, comes in. Every piece - from dresses and tops, to jackets and outwear - is tailored with a larger bust in mind, so you won't have to go up a size to get the right fit.

:: Pepperberry Heritage Check Jacket, £89 (www.bravissimo.com; available September 1)

FASHION FLASH

:: SLOW FASHION

Retailers are losing out on a possible £2.9billion of additional sales a year, thanks to items that languish in mobile shopping baskets. Research from Jumio Inc reveals that 55% of UK smartphone owners have abandoned a transaction due to slow loading times and complicated checkouts, with fashion the most frequently cited dropped items (53%).

:: A THRIFTY READ

Sustainable fashion fans, here's a website to add to your favourites: The Thrift is the new style blog from Barnardo's, focusing on the second-hand bargains and vintage gems that can be found in the charity's shops. Visit www.the-thrift.co.uk

STYLE STEAL

New season drops are landing in stores daily, but the summer sales aren't over yet. Tresor Paris has just added a slew of pretty necklaces and bracelets, reduced to half price or less.

:: Pluie Rose Necklace, reduced to £34.50 from £69 (www.tresorparis.co.uk)