A key method for reducing blur is knowing how to hold a smartphone camera in a stable way. Holding your arms outstretched or far away from your body can make them sway more when photographing. Moving your elbows into the sides of your body can give a bit of extra stability where needed, as can physically resting the smartphone on a stable object.
If you want perfect stability, it is possible to get a tripod attachment that you can slot your smartphone into. Youll probably look a bit silly bringing a tripod out and about to use with your phone, but I have seen and achieved myself some fantastic shots with a tripod in hand.