I'd say, be confident, be casual. Don't word things like an English major. Just say, "Mind if I put my arm on your shoulder?" Or if you feel like not touching them, just give a thumbs-up, or put your hands on your side, or cross your arms...
Limp noodle arms are the bane of picture-taking. xD
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COSPLAYS IN PROGRESS
Utatane Piko - default (Vocaloid) - 90%
Connie (Shingeki no Kyojin) - 0%
Sheik (OoT/SSBB) - 53%