Always brown ground beef just before adding it to the slow cooker. Starting with hot meat helps the mixture reach a safe internal temperature more quickly. By browning the beef first, you also eliminate the extra fat and liquid that would accumulate during cooking.
If you like your chili hot and spicy, use a pound of ground beef and a pound of hot and spicy bulk pork or Italian beef sausage instead of all beef. Cook and drain the two together before adding to the cooker.