Wearing a dressier shirt balances out the casual nature of sweatpants. Button-down shirts easily give sweats a high-low feel, and they distract from the fact that you're in fleece and not denim or chinos. You'll want to avoid wearing a complete sweatsuit while at work. It not only looks out of place, but it also screams, "I don't give a fuck." The same can be said for pairing a simple T-shirt with the sweats, too. Throwing a crewneck sweater or a dark sweatshirt over a button-down automatically makes your entire outfit a little more professional. Bonus points for topping it all off with a camel or navy topcoat.