I usually put a bucket of Ona gel outside the closet door, and another one by the top of the stairs for good measure.
I read somewhere that if you put Ona in the room or near the intake, it can affect the smell of your weed...
My buddy has a big garden and he puts a bucket of hot water with PineSol near the front door. He says it even covers up harvest day smells, but I haven't tried it personally.