I use, 'After you' or 'You first'. If I think that the other person has a right or duty to go first, I may say, "You go first". Often, informally, I take half a step back and/or indicate with a slight movement of the head and a small gesture with the appropriate arm that the other person should go ahead of me. If I am with a modern-minded female friend, I may say 'Ladies first", just to see the expression on her face.
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