During MAGFest earlier this year, I was stopped by an... individual who complimented part of my cosplay. After I gave him some details and thanked him, he told me he was off "to go stare at some more sexy ladies." When I called him on being creepy, he responded that he wouldn't look at underage girls "too long."
No. Nope. Don't be like this guy. This is why we can't have good cons. This is why they HAVE to put up signs that say "Cosplay Is Not Consent" and instructions for reporting harassment to the staff. If you encounter anyone like this (and I know we all have), call them on it. Especially if you're a guy. Cosplay is something that should be enjoyed without fear of being purely objectified or having others creepily leer at you...
Or worse, touching cosplayers inappropriately. I shouldn't have to remind people, but unwanted contact, especially when that contact is in sensitive areas -- It's sexual assault. It isn't funny, it's not "just a joke." It's a crime, and should be treated as such. If you see it going on, alert others (passerby, volunteers) immediately. And if you can, alert a law enforcement official.
It goes for the less gross but still consenting activities as well. Do you like that cosplayer's costume? Want a photo? ASK THEM FIRST. Seems obvious to most people, but it matters. It's also just common courtesy, as it lets the cosplayer get into position for a better shot or fix that loose bit of armor.
So next time you're at a con, or a photoshoot, or just living your life, remember: treat your fellow human beings with respect, ask permission before taking photos or touching people, and oh yeah...
Don't be a creep.