Variations of Side Plank (vasisthasana) are great to strengthen the inner core, shoulders, and sense of balance. Place your weight supporting arm directly under your shoulder, setting it up like a pillar. Whenever practicing variations of side-plank be mindful about your balance to protect your shoulder, and continue to breathe evenly. If any expression of this pose ever gets to strong for you that you have to hold your breath, back up a little bit, reconnect to your breath and then continue to practice from there.