Adding salt, but not too much, is key. Alcohol, like mentioned above, is another key flavor enhancer, and if you add it early most of the alcohol should evaporate as the chili simmers. Coffee or espresso powder is another secret ingredient for some chefs. Again, start with a little, let it cook, taste, and then add more if you'd like.
Soy sauce, fish sauce, and Worcestershire sauce have that "5th sense of taste" or umami. You can try any one of those, but since they're salty, cut way back on the salt you use. Chocolate is another possibility. Use unsweetened chocolate or cocoa powder.