Do you have an active life that involves running, riding bicycles ore skiing? Whatever you do that involves outdoor activities it is not a great idea to bring an iPhone. It's delicate electric circuits doesn't like moisture at all. Many are the customers who sent in their mobile phones for service and then get the reply back: Its damaged by water, No Warranty.
Modern phones have small sensors inside (LCI units, or Liquid Contact Indicators) that gets discolored when they get in contact with water. Here yo can see how it is done on the iPhone ->>
So, put the phone in a plastic bag and then in the pocket? Sure, it is a good idea too. But you maybe like to take pictures with the phone, or have it mounted on the handlebars of your bicycle.
Then you need a waterproof case for the phone. We have tested many units, Incipio Atlas, Griffin Survivor and the Otterbox. However, for us they all fall short. They all lack a bicycle mount. Lucky for us LifeProof have a Bicyle mount for their iPhone cases.
So, you get a Waterproof case that lets you take pictures in the water, like this:
But you also get the option to buy the Bicycle handlebar mount.
The handlebar mount is great, the phone is always in reach for you to take it out to take pictures like this one:
LifeProof also have other options, float cases like the LifeJacket, great when on the open water so you don't loose your phone if it falls over board.