Switch it Off
If the phone still appears to be working after it's taken a dunking, then switch it off immediately.
Don't push any other buttons or keys, as this may cause short circuits within the phone, leading to further internal damage.
If your device has a removable battery then quickly take the cover off and remove the battery.
Whatever you do, DO NOT turn on or plug in your phone. Water and electricity do not play well together.
DO NOT press any keys. This can push water further into the phone. It's best to try to mess with the phone as little as possible.
DO NOT shake or blow into the device. This could also push water into deeper areas of the phone.
Remove Everything you Can
Depending on the smartphone you have various options for removing everything. Some phones such as the iPhone only allow you to remove the SIM tray. Other phones allow you to take the rear case off giving access to remove the battery, SIM card, SD card and so on. Take them all out.
Gently Dab off Any Excess Water
Don't be tempted to rub your phone vigorously to get rid of any liquid. Instead, dab excess water with a kitchen towel.
If water seems to be leaking from any open ports, carefully dab these too, but don't shake your phone.
Get a Freezer Bag and Some Rice
This isn't just the tech equivalent of an old wives tale. Rice soaks up water and it works especially well if you put your phone in a sealable freezer bag and surround it with the stuff.
Cover your phone with as much rice as you can squeeze in, seal the bag and leave it for at least 24 hours, ideally 48.
Remove and Power Up
After a couple of days, give your phone a go. If your phone doesn't turn on, try charging it. If it doesn't charge, the battery may be damaged. You could try a replacement battery, or consider taking your smartphone to a repair shop to be checked by a professional.
If your phone has turned on and it is running as it should, you should still keep a close eye on it for the next few days, to see if you notice anything out of the ordinary. Play some music to check the speakers and ensure the touch screen still responds as it should.
If you have insurance as part of your contract, your network may be able to provide you with a new device, although be aware you may need to pay excess charges. The same applies if your phone is covered under a contents insurance policy.
Options for Clumsy People
If you're prone to giving your phone a soaking, then there are options that'll bring an end to your regular misery.
Some companies sell dedicated waterproof cases for a wide range of smartphones. They may make your smartphone look bulky, but they'll protect it from most spillages.
The other option is to buy a smartphone with waterproof certification. These phones will have an Ingress Protection (IP) rating.
An IP68 certificate means a phone is fully waterproof to a particular depth and for a certain period of time.
There are also IP67 certificates for phones which offer a modicum of protection from splashes and brief dunks in the drink.
Samsung's Galaxy S5 and a number of Sony's Xperia smartphones have waterproofing as standard and can easily be found online or on the high street for a reasonable price.
Apple's iPhones are not waterproof and despite stories of its smartphone surviving long times in water, the company has not made any official announcement about its bestselling product being able to withstand water.