[chmod]

Another good technique would be to carry a large empty box, use a bit of acting to make it look like it's heavy and you are struggling, then walk up to the door as someone is going through. Get their attention as they pass through and they should naturally hold the door for you.

Even Kevin Mitnick explained this in The Art of Deception.

Thanks for that I knew I had read that from somewhere but couldn't for the life of me remember where, although I don't think I read the entire book. I'll have to find it sometime