It might not seem very exciting, but DON'T skimp on flanking practice!
For day-to-day farming tasks you might be able to work around your dog's shortcomings, but when the unexpected happens (as it surely will) you, your dog, or more likely your sheep, will run into trouble if the dog is reluctant to flank freely in both directions.
Why Your Dog Should Flank Both Ways relives an incident that demonstrated the value of a versatile dog. Happily our emergency was on cosmetic and economic grounds, but anyone who keeps sheep near railway tracks, water, roads, or neighbours' gardens should always be in a position to retrieve their sheep safely, and minimise damage.
The older the dog, the longer it will take to correct the habit of a lifetime, but it CAN be done.
For a simple but essential training exercise to correct or prevent one-sidedness in your dog, watch the Backwards is the Way Forward tutorial (DVD volume 2). Or to see one-sidedness "in action", look out for Scylla in the Bronwen and Scylla series.