I was taught by an 98 year old neighbor to prune the star magnolia with a heavy hand. Open it up, remove any crossed branches, let the sun shine between the branches and prune it to allow the branches to go outward and upward, and tell the tree who is boss. (That's what he told me). I usually pruned just after flowering, and removed about 1/2 the growth; don't know if that is correct and by the book, but it worked for me for many years.
