It's one of my help guides, but it's so important that I fel that I should remind you all:

YOU MUST MEASURE YOURSELF TO CHECK YOUR BRA SIZE REGULARLY!

How often really does depend on your circumstances and body shape, but make sure you do it at least every year just to check. If you are losing or gaining weight, or if you've changed your exercise regime, you will need to do it more often.  You can start right now, by looking closely at the bra you are wearing right now, checking all the key areas to be sure that it is fitting and working for you as it should.  A bra who's elastic has given way due to age will be just as inefficient as if you wearing the wrong size, so make sure you keep an eye on the age of your bras too. 

Let's get on to what you need to do:

The most common mistake is women wearing a band size that is too large, and therefore not offering the support it should. 

This is how to make sure it is correct:

Step 1: Grab yourself a nice soft tape measure with inches on (preferably not a rusty steel rule from the garage)

Step 2. Underbust Measurement - Run a tape measure all the way around your body just underneath your breasts and take a measurement in inches. Make sure the tape measure is horizontal and snug. Your arms should be as down as you can make them.If this measurement is an odd number, try the two sizes around it, For example, if you measured 31 inches (78.7cm), you should start trying on bras in a 30" and 32" size. If it was an even number, start with that size band and see how you get on. Every bra is different, but this gives you a starting point. NOTE: Some bra fitting guides and calculators will tell you to add four or five inches (10 to 12.5cm) to your underbust measurement, but this is outdated advice. The "add four" method was popularised by Warners in the 1930s when bra design was in its infancy and it simply doesn't work so well with modern bras.

Step 3: Since everyone's breasts are different in terms of shape (even between your own breasts), the most accurate way to determine your cup size is by using your current bra size as a starting point. If you don't have an existing bra, take a look at the sized cups for your band and give the one that you feel will be the best a try.  The cups are sized relative to the band, so if you were to try a smaller band size but keep the same cup size, the cups would be too small - if you go down a band size you need to go up a cup size. If you decide to go up a band size, then to maintain the cup size you will need to go down a cup size. 

Step 4: Put the bra on around your body first doing it up on the LOOSEST SETTING. The elastic in the bra will loosen with age, so you need to be able to tighten it through it's life. It should always start on the loosest hooks. Wiggle your breasts into position, yes wiggle, it works! Tighten the shoulder straps so that they do not dig in and hurt, or slip off the shoulder easily. The band around the body should be taking the main weight of the breasts.

Does the bra really fit? Things to check:

Does the bra feel tight around the body? If so, good! It may feel too tight, but this is because we are used to wearing bras that are too big for us. It should feel tight, but not dig in. You should be able to get 2 fingers side by side between the bra and your skin, and it should have a little bit of give. Check that the weight of the breasts is well supported so the shoulder straps are not digging in at the top. It should of course feel comfortable.  

How about the cup? Are you well contained in it?! You need the cup to hold you in nicely, without the breast spilling over the top.  We also don't want a gape. Some bras will give a little gape and that is more down to the shape of the cup or the shape of your breasts, but this should only be a small gape. Any more than that, and it's not supporting you enough.  You also need to be sure that the underwire follows the correct curve of your breast. Make sure that your breast does extend beyond the underwire under your arms, and if it does, it means that the cup is not big enough for you or that the shape of the bra does not work for you.

They are all the key points. The main thing is to be honest with yourself.  Many fashion bras are designed for a night out, and not everyday wear.  Go for those pretty bras, but also make sure you get a bra / bra with enhancer or breast form combination that works for you with full support for everyday use. Your breasts and back will thank you for it when you are older.