I think to look approachable you have to look sort of energetic, so opening your eyes a little wider (not like really wide, but just not sleepy-looking) raising your eyebrows a bit (looks friendly) smiling here and there, and kind of standing up straight and not having your shoulders too far forward (it goes with the energetic look, and hunching over kind of looks like you're trying to hide or something) and if you can manage it, meet people's eyes more often. You don't really have to look them in the eye much, but a couple glances per conversation would be good. The energetic, friendly body language should do most of the work.
...at least, this is what has worked for me.
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avatar photo: Mount Rainier
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