If you’re not wearing a costume then I suggest you make sure you wear clothes suitable for the weather (ie. if it’s freezing then wear lots of layers and wrap up warm – you can always take layers off if you get warm). If you have an anti-circus t-shirt then wear it, if not then dress fairly ‘bland’. For example, I dress in a goth style most of the time but for protests I try to look fairly ‘normal’ to encourage people to talk to me.
Also, take a drink: if you’re chanting and talking to people a lot then you’ll get a dry throat!
Make sure you read up all about Ringling and circus cruelty so you’re ready to answer any questions that people have.
Just get involved with everyone else, smile, be friendly and good luck!