Depending on your budget you can either have your teeth whitened by a professional or go for an over-the-counter product.
Each approach has its pros and cons.
Professional whitening by a dentist can be pricey but you're more likely to get a good result and you also get advice and products specifically for you.
Over-the-counter products range in price and can be a good place to start, but results are varied and the necessary steps can be hard to maintain.
What can I expect from a dentist?
General practitioner of dentistry Dr Ray George, of Raymond J.
George, in Dunedin, offers three options for whiter teeth.
Power clean
This is a simple way of cleaning the teeth and works like a mini water blaster, using a modified form of baking soda, air and water to remove superficial stains caused by things such as coffee, tea or red wine.
This cleaning is done in the office and can be repeated as often as desired.
Cost: Around $85.
Dentist whitening
This procedure is done in office and uses a very concentrated carbamide peroxide bleach, which is carefully applied by specialists, to whiten the teeth.
It's very safe as the only thing that touches the nerve is oxygen, says Dr George.
This treatment is good for people who need results in a hurry, as it takes less than two hours to complete.
Cost: Around $899.
Take-home whitening kit:The benefit of this option is that it's more economical than in-office whitening while achieving the same results - it just needs a little longer.
Moulds are taken of your upper and lower teeth and then trays - like mini mouthguards - are made.
Because the trays are custom-made, they match your teeth exactly, which should ensure the whitening gel goes where it's needed.
This initial kit of mouth trays and tubes of gel is usually enough for the initial bleaching and then a second application 4-6 months later.
The trays can last up to 10 years, so to carry on bleaching you just need to buy more gel.
The technology behind take-home whiteners is getting better all the time, says Dr George.
In the 1990s you had to wear the tray all night for up to two weeks, now it's just an hour for 2 to 3 days.
And it's safe.
Cost: For initial take home kit, about $599.
Single tubes of gel, about $28.
How white will my teeth go?
While whitening may conjure up images of Tom Cruise flashing his super pearly whites, the reality for most people is far more subtle.
Hollywood teeth may look great on camera but in real life you'd look like Mr Ed, Dr George says. The true aim is to look better in general.
You want people to say you look good, not `God you've got white teeth'.
Whitening contributes to an overall sense of well-being and positive self image.
According to Dr George, it's impossible to over-bleach, as teeth reach a natural end point where they won't change any more.
This end point is hard to predict, as everyone is different.
"We explain the shade thing carefully, so people have realistic expectations.
Our experience is that we get a lot of people about two to three shades improvement on the dentists' scale.
It looks great, but not so obvious as to have people comment.
How long will the result last?
It's important to note that all bleaches regress slightly after six weeks before settling.
After this, results vary depending on lifestyle choices such as smoking or drinking coffee.
There's no point in bleaching teeth in isolation, says Dr George.
You need to integrate it with a healthy lifestyle.
What about fillings and crowns? Fillings and crowns will not change colour during whitening so the best way to get a perfect smile is to bleach the teeth, let the shade settle then replace any fillings and crowns.
What about over-the-counter products? While the range of products is a little daunting - gels, pens, paint-on options - many pharmacies in Dunedin have only one or two whitening products in stock.
In the name of research, I tried five-minute Natural White (about $10 from pharmacies), which is a mouthguard and gel.
This mouthguard is obviously much more generic than those custom-made by a dentist, as they need to fit everyone.
I had to apply the gel to the mouthguard and wear it twice a day, for five minutes at a time for no more than two weeks.
It's hard to measure the results myself but when I compare my front teeth with those further back the shade is definitely whiter.
As they're sold over the counter, these products have a lower concentration of peroxides and therefore take a longer time to work.
But do they really work? Dr George says results can be fair to good with some of these products, but the main problems are consistency and application.
People start off with a hiss and a roar but seem to flag it after a week.
Most people struggle with being conscientious with it.
There are also problems with application and inconsistencies with method.
What about whitening toothpastes?
In controlled experiments they will work, says Dr George.
"In the real world I wouldn't like to say.
"They won't do any harm if they're mainstream brands."
Can everyone whiten their teeth?
No.
Pregnant women are advised against whitening, as are people under 16.
Dr George won't whiten teeth of people under 20 as the enamel on teenagers' teeth is soft and there is a risk of damaging it.
They don't usually need to anyway, he says.
And the older you are, the harder teeth become, as well as more stained and darker.
"I've yet to find anyone medically who couldn't do it."
What are the worst things for staining teeth?
Smoking: Smokers' teeth are harder to whiten as they have both superficial and intrinsic stains (inside the tooth).
Whitening works but not as well, says Dr George, and they need to get it done more regularly.
If people stop smoking, they will get better results.
Smokers should also avoid smokers' toothpastes.
These are very abrasive.
They make the teeth look white in the short term, but they are actually taking a layer off.
If you keep using them, the enamel becomes thin and you start to see a dark inner core of tooth (dentine).
It ruins your teeth.
Best advice: Stop smoking, use mainstream whitening toothpastes and get a professional whitening clean.
Ageing: Sad to say but ageing is the biggest changing factor.
Young teeth are naturally a whiter colour than those of people in their 50s and 60s and the benefits of whitening appear to last longer.
While it's harder to whiten older teeth, it is still possible.
The whiteners bleach the protein in teeth and, as older people have less protein, there's less protein to bleach.
Food and drinks: Coffee, tea, red wine and coloured foods, such as beetroot, will stain your teeth.
Any heavily coloured food which would stain your clothing would stain your teeth, says Dr George.
The good news is that these superficial stains can be cleaned up.
Genetics: Genes play their part, so look to your parents and grandparents.
Some lucky people just have whiter teeth than others.
