Timothy McVeigh, American terrorist, preferred a more groomed look, and white T-shirts. If you don't want people to think you are a terrorist, grow a beard and don't wear white T-shirts. Maybe you shouldn't be a man anymore, as well. Just to be sure.
Andrew Joseph Stack III, frequently referred to as a "tax protestor", flew a plane into a government building. Clearly terrorism. He favored plaid button downs. Remove any plaid button downs from your wardrobe immediately if you don't want to freak people out on planes.
Hutaree, famous for a multipart plot to kill police officers by the dozens, made it just threatening to be a woman as a man, by including a woman in their group. They preferred to wear colored T-shirts and camouflage.
So, just to review, if you don't want people to mistake you for a terrorist, shave your beard and cut your hair, don't be clean shaven, don't be a man, don't be a woman, don't wear hoodies, don't wear plaid button downs, don't wear white or colored T-shirts and don't wear camouflage.
Oh, wait, silly me. All you have to do is not be brown in America. American nonMuslim terrorists don't exist, you see.