Jeans or sometimes cargo pants (for the extra pockets), T-shirt with or without an old windcheater, casual shoes or sneakers. The jeans and pants are OK being cotton, but the rest are not the best for safety.
I've thought about getting Nomex or flying overalls, but Nomex is pricey and uncomfortable, and flying overalls seem a bit over the top in an enclosed cockpit. Maybe if I had a Tiger Moth.