Soft resetting your iPhone can fix a lot of issues and is something that you should consider from time to time in order to clear the memory.
Close all Apps by double tapping the Apple Home button and then
iOS7 + Swipe Up on all Apps to close them
iOS6 or lower hold your finger down on an App icon until all icons start to shake along the bottom and then close them all by tapping the x on each.
Next, hold down the top button while holding down the Apple Home button until the phone restarts and displays the Apple Logo.
You have soft reset the phone.