How to prevent an iphone from getting slow?

My friends iPhones have gotten all slow after a while, and i just got mine and its super fast. How do i prevent my iPhone from becoming slow? What causes iPhones to become slow?

Don't have too much pictures. That will take up memory and cause it to be quite slow. Also with the new update, if you double-click the home button the apps that you have recently used will pop up swipe them upward it will stop them from running.

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