Essentially, as people have said above, if you know you are going out to have it, then suit up. If not, gloves, decent leather / textile jacket, good shoes / boots, and a decent pair of gloves and of course lid.
Whatever you feel comfortable and safe in, imo, is fine. I do get VERY irate at people I see on bikes in shorts and t-shirts over the summer though. But, that's their problem frankly. They'll realise why they shouldn't do it the moment they contact with the tarmac...