I also find using the volume up button can cause blurring. You will tend to use it one handed and are not as steady, usually.I'll add this:
After composing a shot and locking the exposure/focus, tap and hold the shutter for a moment to steady your hand. Release your finger to take the shot. The movement of tapping itself can cause blurry pics.
...and using earbuds has the advantage of silencing the camera click to help keep your people shots candid.Another good way to keep the shot steady when triggering the shutter is using the earpods/earbuds like a remote shutter release unless you have to use both hands to keep the phone steady.