I found a bunch of these on the internet, and gather them here, with credit to the authors.
STRENGTHEN THE HINDQUARTERS:
To strengthen the hindquarters, teach your dog to sit up, stand, and move back to the sitting up position without their front feet touching the ground. Start with the dog in a sit. Raise a treat slowly above her head to get her to sit up. Mark and reward. When she has this step down, have her sit up and then bring the treat straight up so she stands. Mark and reward. For the last step, very slowly lower the treat so she sinks back down onto her hindquarters. For longer-backed, out of condition, or heavily-built dogs, you can teach them to sit up while bracing them against your legs (they will have to sit with their backs facing you). Dogs with very long backs like Corgis may always need support to sit up safely.