how can I make a smooth walk cycle?

Yup, pin/bone positions are often the cause of this. Take a look at some of the template puppets like Walkbot and see where those pins are placed. Also, look at the tags section where you see the UI of the character with the joint placements. That will also give you an idea for where the pins should properly be placed.

Could you share a screen shot or video? One thing to watch for is the relative position of the head and neck handles. You might like to try moving the neck handle around a bit to see if it improves posture.