Some doors only open when you push on them. It can be rude to ask for a free gift, but some things require request. Know the difference.
When it is time to knock on the door, knock gently and kindly. Wait until answered. Knock again if no one hears. If no one is home, go away and come back to knock again another day.
Shut doors open with two keys: patience and charm. You have to keep knocking, but you must also smile while you wait. You never know who may be inside looking out, deciding before giving an answer.