I am not sure if there is one that isn't awkward.
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They get hit on by fat, old white men stuffing themselves full of chicken wings all day. Their sense of self worth is only marginally higher than a stripper's. Barring you looking like gollum, taking a picture with you is by far not the worst or most awkward part of their day.
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Show us the picture once you've taken it.
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Are you dining alone or with friends? If alone, it is VERY awkward. If with a group, just something along the lines of "you know, we come here often and you've been such a fun waitress, can we get a group pic for Foursquare?" Also, tipping well goes a long way.
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"Hey babe, mind if I objectify you even more? I need something to show to the kids."
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I'm sure they are quite use to it.
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In your bedroom.