Benefits of planking
Planks build a strong core and increase upper strength.  Additional benefits include improved balance and posture.  When done with correct form, planking can bring substantial results in less time.  Consistency and progression are KEY.  

Find your starting point
To start planking safely, choose a variation that fits your fitness level.  If you can hold a plank for 1 minute, consider progressing to the next variation.  NOTE: If you have injuries, consult a fitness professional prior to beginning your planking program.

1. Incline forearm plank (easiest)
2. Forearm plank on knees
3. Forearm plank
4. Full plank (more difficult)

Proper Form
1. Shoulders over elbows or hands.
2. Tucked tailbone, squeeze glutes or buttocks.  You should feel your abs tighten as well.
3. Look ahead slightly to keep spine neutral (see photo below for head position).
4. Toes about shoulder width apart or less.

NOTE: Head position

Progressions

1. Incline Forearm Plank 

2. Forearm Plank on Knees

3. Forearm Plank

4. Full Plank