We've all heard that most women are walking around in the wrong size bra. "That's hardly a surprise," I hear you say, "considering how hard it is to work out what does fit!"

Yes, finding the right bra can be frustrating, but if you learn a few simple rules, it can be done. If you know how to try on a bra properly, and can spot the tell-tale signs of a bra that's never going to fit, you can find a comfortable bra that will feel good as well as look good.

The problem is, bosoms come in all shapes and sizes. Short of custom making, bra makers can't possibly make a bra to fit every single variation. The width of the back, the shape of the breast, the size of chest and the rib cage - all combine to create an amazing number of variations.

Bra manufacturers have to decide how to average all that out in a range of sizes that usually only goes from A to D, and they don't all make the same decisions. Some of them will choose models shaped like you - others won't.

That's why you'll find some brands fit better than others. For instance, I would love to wear Marks & Spencers bras to go with my M&S undies, but they don't fit. Ever.

Actually, I'm a 36DD so there aren't many bras in the average department store to fit me. Which brings me neatly to my next topic - discovering your size.

The easiest way to determine your size is to go to a store and be professionally fitted - but I'm a little cautious of these lingerie ladies. These days, they're often employed by a bra manufacturer, not the store, So naturally they want to sell you their own make - and if it's a brand that's not your shape, you could walk out the door with a bra that fits just as poorly as your old ones!

If the fitter doesn't have a tape measure, don't be concerned. Go to a top-flight company like Rigby & Peller of London, and you'll find their fitters never use one! Instead, they assess your size by eye, then try bras in a range of sizes to find what works. It's really the best way. Bra sizes are based on only two measurements - your rib cage and how far your boobs project over it. Get a few women together and you'll see there are many other variations in shape, position on the chest wall, depth of the back etc, so those measurements are only a small part of the story.

If you're trying to work out your own size, you need to start somewhere - and a tape measure is as good a place as any.

The number in your bra size represents the size of your chest. There are two ways to measure it. You can measure under your bust, around your rib cage, and add 4 or 5 inches (whichever gets you up to an even number). Or you can measure around your chest directly under your armpits (don't add any extra to this figure).

In both cases, keep the tape in a straight line (look in a mirror or get someone to help you) and breath all the way out, or the band will be too loose.

The letter in your bra size represents your breast volume, but that's problematic. Ideally, you want to measure while your breasts are supported in a nice rounded, lifted position. If you measure without a bra, your breasts may be drooping and the measurement you get will be too small. If you wear a bra that's too small, your boobs will be squashed. If your breasts are already uplifted and sitting exactly where they should be - what do you need to measure for? Sounds like all you need to do is read the label on the bra you're wearing!

However, for what it's worth, here goes. Measure around your rib cage under your bust again. Don't breathe in, we want your resting measurement. Now measure around the fullest part of your breast. What's the difference between the two? Every full inch is a cup size. So one inch difference means you're an A, two inches means you're a B, and so on.

Armed with this information, you can head to the store with at least some idea where to start!

If you can find a store that lets you take a pile of bras into the changeroom, go there! Otherwise, be prepared to make several trips: choose a top you can throw on quickly without a bra, so you don't have to do a whole lot of dressing and undressing all the time.

Pick a selection of bras from different makers - not just in your theoretical size, but in sizes either side. Now, before you try anything on, take the bras off their hangers and loosen all the straps to their fullest extent.

If you've loosened all the straps, it's time to try the bras on. This is where you're going to get frustrated, because you'll find many of the bras that say they're your size are lying!

Be patient, but be ruthless. The first two tests below are deal-breakers - if a bra fails one of these tests, take it off immediately. Move on to the next bra.

Put the bra on, fasten it at the back on the loosest hook, then lean forward and lift each boob, and let it fall into the cup.

The band test

Look down between your boobs. If you can see daylight between you and the band where it crosses between your breasts - even a tiny chink - take it off. It will never fit properly, no matter what you do, because it's at least one size too big. Note I'm talking about the number, which relates to the band, not the letter, which relates to the cup.  So if you're trying on a 36 and you can see daylight, you need a 34. 

The cup test

Once you've found a bra that passes the band test, put your thumbs on the sides of the bra under your arms (pretend you're about to to do the chicken dance). Feel where the edge of the cups touch your skin. Is that breast tissue you feel under the wire? If the answer is yes, the cup isn't big enough. The outer edge of the cup needs to be sitting off the breast, on the flesh under your arms. Once again, if this test fails, take it off - there's no point doing the rest, because that bra is never going to fit. To find a cup that fits, go up one letter at a time (e.g. if you tried on a 34B, try a 34C).

The ski jump test

Now adjust the straps before you do the third test. The taller you are, the less likely you'll need to adjust them - because this adjustment depends on the distance between your shoulder and your boob. A lot of women make the mistake of tightening the straps to try to make too-big cups fit. It won't work, it will only make them pucker! If you have to tighten the straps more than halfway, ask yourself whether you've picked the wrong cup size.

Stand sideways to the mirror and look at your profile. Your breasts should have a lovely smooth silhouette from neck to nipple - no gaps and no bumps. Imaging a ski jumper flying down that curve - would he shoot off the end of the run to a great jump, or would he come a cropper on the mogul just above the cup line (or alternatively, disappear down the gap between you and the bra cup?).

The overflow test

The last test is the back. The band must be sitting flat all the way around, parallel with the floor - not up between your shoulder blades! It shouldn't be cutting in and causing bulges above and below. However, bear in mind that if you're carrying extra weight, you'll get some overflow even in your correct size. If that applies to you, styles with a broader band will improve the look.

It may sound like a lot to remember, but think of it like this:

Straps out! Centre front - underarms - overflow - back. Not so hard, is it?

Follow these rules and you will find your perfect fit, but it will take a while. Measuring myself with a tape told me I was a 38C, and it took a lot of trial and error - and, eventually, a trip to a very upmarket bra shop with a professional fitter - to confirm that I was really a 36DD.   It's been worth it though, not to have wires digging into me, straps cutting in or backs riding up. Ah the relief!

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All text copyright Marisa Wright. Photos from Flickr - "Bra fastening" photo thanks to Mangku, "Bra Shop" photo courtesy of Daquella Manera.