I just worse a t-shirt with a jacket on top (found that I went from freezing very quickly. Jeans for comfort and smartness. Tended to wear muckboots or high riders if I was riding.
Depends on the type of show, at RC show jeans and smartish jacket fine, at our local show which has qualifiers for PUK etc we are told to dress smartly (we were allowed to wear trousers 3 years ago!) and a hat.