Felix, I totally second the Nazi salute thing. That is pretty much just as bad as coming to Kumoricon in a KKK uniform. What happens in private photoshoots or in your hotel room, I really don't care, but if you're in a public area of Kumoricon, please keep anything unacceptable away.
Also, please keep your voice down. Screaming, shrieking, and yelling is largely very annoying to everyone outside you and your friends. This is double-y important when it gets dark out. People do sleep at cons. I know, it's shocking, but it does happen.
If you are stumped how to do something, by all means ask the forum. There's lots of people who can help out here.
If you're on your own, think outside of the craft/material stores. A lot of con supplies and ideas can come from walking around Lowe's or Home Depot.