Make it a series and:
-Plan out the set of Hogwarts from the beginning on, no changing of locations or add ons
-more out of Hogwarts stuff include all Weasleys
-if there are changes from the book, make sure it's adding to the story at hand, not cutting part out
-stick to the characterisations of JKR (Not Ron as a sidekick, make Snape an a** again etc.)
-ONE director, keeps it steady