We're not encouraging public alcoholism or anything, but this tip demonstrated by Chris of college lifestyle blog Hack College shows you how to create a soda can sleeve to smuggle around your beer:
Basically, you just cut the top and bottom from a soda can using a box-cutter and then cut a seam. Add a piece of tape to the seam, slide the sleeve over your beer can and presto!
Obviously, this sleeve won't work if someone actually grabs your drink out of your hand. And if you feel the need to sneak your public boozing around, then you might have bigger problems than you think.