When it comes to Paris, and New York style, the running theme is that everyone looks effortless, but really chic. I think the trick to that is being comfortable in the clothing but making sure it's high quality. If you want to look casual, feel comfortable, maybe it's a great pair of jeans and a cashmere sweater. That's the type of thing where you'll look casual and effortless, but the quality of the clothing will really shine through.
Another thing to keep in mind is making sure you're wearing classic staples. You can be really casual in, again, a pair of jeans, a pair of flats, and a white T-shirt, and a blazer. You're not going to look sloppy. You want to make sure that everything fits well and it's elegantly cut. So, if you're buying jeans, make sure they fit really well, that they are flattering, and then wear them with a neutral flat, maybe a black ballet flat, or even a nude flat. Then you can just buy a Hanes, white V-neck T-shirt with a really, really well-fitted linen blazer. That's an easy way to put together an outfit that takes no effort. You'll feel so comfortable. You're going to look chic and elegant. You're not going to look dressed down, and you're not going to look inappropriate for the work space.
Other things to do are scarves. A silk scarf is a really easy way to dress up an outfit and add that European (?). That's a great way to bring an old school glamour to something that's really, really simple. I think the key things to keep in mind when you're dressing in a way that you want to feel comfortable but still look good, again it's fit and it's quality. As long as what you're wearing fits you and as long as it doesn't look cheap and maybe feels really good, that's the best way to come across as effortless.