Whilst searching for some alternatives to support an idea of mine regarding decarbonating, I found a somewhat amusing thread:
How can I de-carbonate soft drinks?
(from the boards of "The Straight Dope").
Based on ideas mentioned there and another one of my own: Pour the drink into a larger container to increase surface area, i.e. a cake tin.
Leave for a little while, before repoured into a drinkable container Put the drink into a blender, and give it a spin.
A less amusing option, is to stir with a spoon for a few minutes Sift the drink into another container.
That is either use an ordinary sifter/sieve/strainer, or use something like a coffee filter Either of these options should agitate the drink and decarbonate it.
In terms of practicality, I would opt for the latter one.
Especially if at office or at home.
At office you need to a get a little sifter, but they are readily available.
lifehacks.stackexchange.com/users/4490/holroy
Adding a teaspoon full of sugar will get rid of carbonation quickly; you will of course have a slightly more sugary drink.
lifehacks.stackexchange.com/users/2431/Minnow
Use a vacuum wine saver.
This is a product that has a bottle cap with a valve and a pump.
It is intended for pumping the air out of the top of a wine bottle to keep the contents fresher
but if you've ever actually made wine you'll probably know that you have to degasify it before bottling and you can use one of these.
Put the cap on bottle with the drink in and keep pumping until you see no more bubbles coming out of the liquid
lifehacks.stackexchange.com/users/5304/Dave
Put a frying pan on the stove, pour the soda in, and crank the heat.
The frying pan will preferably have ridges in it ( it wont be an exactly flat surface ).
By the time the soda is boiling, it will be pretty darn flat ( a minute or so? ).
You don't really need to boil it, bringing it to 60 degrees Celsius will probably suffice.
A carbonated drink is one that has had carbon dioxide ( CO2 ) dissolved into it.
This is generally done by forcing CO2 into it via pressure, and this is the reason why air ( CO2, in fact ), is pushed out of the bottle when you open it.
The word "dissolved" is key, as it speaks about the solubility of the CO2 in the liquid.
Temperature greatly effects this solubility, as the temperature increases, CO2 solubility decreases.
If you think about why boiling water creates bubbles, this makes sense.
Oxygen is not as soluble in hot water as well.
Water near its freezing point can contain the most CO2.
As the temperature increases, this amount drops significantly.
However, it is not a linear drop, and by about 50-60 degrees Celsius, the curve is leveling off.
As KalleMP states, you don't really have to bring it to a boil.
However, what kind of life hack is it if you're holding a thermometer in your hand?
:)
There is also the fact that a freshly opened bottle of soda contains more CO2 than the atmosphere, so the CO2 will escape even from ice cold soda.
Increasing the exposed surface area of the soda will increase the rate of this escape.
The bubbles that form inside your glass are doing so at places that actually contain microscopic air bubbles ( think porous ).
The CO2 will rapidly collect in these points, and larger bubbles will form.
These are the bubbles that you see coming off the side of your glass.
So, high temperature, surface area, pores...
I'm going to go with boiling it on a frying pan.
lifehacks.stackexchange.com/users/4700/Carl
Being a big tea-drinker, I have 1,5l glass containers with wide openings.
Simply pour your drink into a large container from a decent height.
A bottle of half a liter will bubble up to the top, but once settled down again will be devoid of carbon dioxide.
Depending on the drink, container there might still be a tiny bit of fizz.
lifehacks.stackexchange.com/users/8185/zaruvi
My girlfriend stirs her drink with a raw wooden spoon.
This can also work with a bamboo stick but since it has less surface area, it takes a little longer.
It's still quick enough, after about 3-4 stirs it will be completely decarbonated.
A warning, however, don't fill your glass to the top, because when decarbonating, it creates bubbles and there's a chance the glass will overflow.
I couldn't believe how well it worked the first time I tried it.
I don't know exactly why it works so well, so if someone can explain it, please do.
lifehacks.stackexchange.com/users/19327/Elof W
I prefer a lot of fizzy drinks when they have gone flat. What is the most efficient way to decarbonate them?

I've repeatedly shaken and opened bottles, left caps off, stirred, but I wonder if there's a more efficient way.
What is the quickest way to de-carbonate a drink
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<food><efficiency>
lifehacks.stackexchange.com/questions/8114
lifehacks.stackexchange.com/users/47/James Webster
There are many different ways to decarbonate a drink. You could first consider boiling it or raising it to around 60 degrees Celsuis on the stove. Alternatively, you could give it a stir for a few minutes with a teaspoon or even a wooden stirrer. You could also consider using a sifter/sieve/strainer, putting it into a blender or pouring it into a larger container to increase the surface area. Other options include adding more sugar or using a vacuum wine saver. The options are to leave the drink to stand in a large container, put it through a blender, stir with a spoon, add a spoonful of sugar, use a vacuum wine saver to pump out the gas, warm it in a pan or pour it into a large container from a decent height or stir with a wooden spoon or bamboo stick.
Pour the drink into a large container and leave to stand, put it through a blender, stir with a spoon or pour through a strainer. Add a teaspoonful of sugar. Use a vacuum wine saver to pump out the gas. Warm it in a pan to release the CO2. Pour into a large container from a decent height. Stir with a wooden spoon or a bamboo stick 3-4 times.