Eye contact is the key. If you like the look of someone at the other side of the room - repeated and slightly prolonged eye contact with them should get the message across. Obviously you need to smile at them once in a while too, but smiling without eye contact may even make you appear more unattainable - like you're having such a good time with your friends, you're not interested in getting approached by anyone.

Also - try going out with your hair up. Exposing the neck and the wrists is a sure fire 'come hither' body language tell.

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