I use my left pinky to hold the base of the phone. Lots of people with bigger phones use one of those stick on things on the back and hold it with their index and middle finger.

+1 for the pinky phone shelf. Worked better when phones were smaller and lighter now I have a popsocket on the back and hold it either between the middle and index or middle and ring depending on what I'm doing. Sometimes with middle and index the pinky comes out again to stabilize the phone too.