The bridge is a fundamental exercise to use for spine mobility. The slow purposeful movement will greatly assist in freeing vertabrae and lower abdominal strength comes as part of the benefit too.

The exercise:
The bridge starting position

Starting position:

Lay on your back with legs up and knees bent so that your feet are on the mat about a thigh length from your buttocks.
Ensure that your lower stomach muscles are tight and hips are in the forward position.
Your feet should be slightly apart.

The movement:
The bridge finish position

Take a deep breath.
While exhaling slowly raise your hips until the hip joint is fully extended.
Ensure that your back does not arch during the movement.
while slowly moving, feel each vertibrae release and move to the next one.
When the movement is complete slowly inhale while lowering yourself to the starting position.

Repeat about five times.

Notes:

Ensure that the neck is relaxed during movement.
If any pain occurs stop the exercise immediately.