We've spoken before about the importance of a well-fitting bra and how to identify when things aren't right, but why is it that even women who know the rules seem to forget them when it comes to buying a bikini?

A bikini top has the same role as a bra - it's there to cover and support your bust whilst looking good. So do yourself a favour and think before you shop for swimwear. This is not just about fashion, but fit as well. Trust me, your boobs and body will thank you!

Here are the biggest signs of a badly chosen bikini and how to avoid them. The images are proof that even some of the world's most beautiful women can get it wrong!

1. Underboob!

Holly Madison Playboy underboob bikini picture

Oh Holly. Holly, Holly, Holly. Just because they're fake doesn't mean they don't need a decent bikini top. If you've paid for them, look after them!

If you can see flesh poking out from under the fabric of a bikini top, it's a sure sign the cups aren't right. The bottom of the bikini (whether it's made from underwiring or fabric) should sit directly under the bust, flush to the skin, not halfway up the boob or hanging away from the chest. Think of the bra fitting rules we've spoken about before, and apply them here too. The band should take most of the strain, not the straps.

2. 'Fits all' cups

ill fitting bikini too small polka dot

One of the biggest issues with modern bikinis is the 'one size fits all' approach. Do all women who are a size 10 wear the same sized bra? I'm guessing not, yet there are plenty of brands out there selling bikinis by dress size not cup size. This might be absolutely fine if you happen to fit the average, or if the bikini doesn't have 'cups' per se, but you have to try it on. The result is often more like the above picture, where the bikini top is clearly far too small in the cups. Look at how the centre gore sits away from the skin, not to mention all the excess breast tissue poking out either side. Ouch! I'd recommend nobody buys a bikini with a bra-style top unless it comes in proper cup sizes.

3. The wrong band size

aubreyoday_toobigbikini.jpg

I don't even know where to begin with this one. The band is around her waist! Poor old Aubrey, if she breathes in, her bikini will probably fall down completely. This picture is a lesson in choosing a bikini top that's the correct band size for you. This one is probably 4 or 5 inches too big for Ms O'Day. The underwires should sit directly under the bust to push it up, with a band that provides support. Instead, we have a band that sits far too low down, giving no support at all and creating a gap between the breasts that you could drive a tractor through. Some of us naturally have wide-set boobs, but that just makes the need for support all the more important. The irony is the cups are probably the right size, if only she hoiked the whole thing up a bit!

4. Oh no, bandeau!

Christina Milian in a bandeau bikini underboob ill-fitting

Unless you have small or particularly perky boobs, bandeau / strapless styles (particularly those without underwires) can cause all kinds of problems. Look how Christina Milian's bikini has not only ridden up giving her underboob, but also twisted too far to one side. She's about 10 minutes away from losing all modesty! With no straps to hold it in place, a bandeau style will quickly swivel round and expose bits you perhaps didn't want people seeing. These designs are great for sunbathing (no tan lines) but pick a strapless style with underwires if you've got a larger bust and you plan on moving about a lot. Otherwise, you'll be re-adjusting every five minutes.

5. The big dig

bikini_digging_in.jpg

It's a very good sign that it took ages to find a photo to illustrate this issue, as it suggests most of us know the obvious - that a bikini needs to fit to look good! But diggy-in bikini bottoms are perhaps more of a problem in the 'real' world, where people have more fleshy bits than the toned models and impossibly slim celebs we see photographed in bikinis.

It's very tempting (especially since swimwear stretches over time) to go for a very snug fit when you shop. But swimwear that's too small is both unflattering and uncomfortable. Forget the label and buy briefs that fit. To avoid them digging into flesh too much, try a bikini brief with a foldover waistband or a frilled or skirted section. It's also worth ditching the £10 bikinis and paying a little bit more for a heavier, higher quality fabric. It'll last longer and fit better in the long run.