Yoga in general is full of core strengthening poses. Most core work in yoga has the added benefit of toning and strengthening the shoulders and upper back, as well.
These are some great prep poses for your headstand!
Dolphin Plank (Forearm Plank)
Ananda Balasana (Happy Baby)
Reclined Side Leg Lift
Bhujapidasana (Shoulder-Pressing Pose)