How to Fire a rifle from either shoulder w/ a Highsmith drill
Check out this video tutorial from Viking Tactics on how to fire a rifle from either shoulder.
The Highsmith Rifle Drill is designed to push shooters' limits on strong side to support side transitions with the rifle, proper barricade kneeling techniques, and lateral movement while firing - from both the strong and support side.
This is a personal favorite. Transitions between strong and support side take quite a bit of practice - as does accurately (and comfortably) firing from the support side. Considering the alternative (exposing half of your body to hostile fire), and consider practice on this one to be time well spent.