Deciding when to replace shoes that still look fine but have many kilometers

My running shoes still look clean, but they've accumulated a lot of kilometers and the cushioning feels less lively than I remember. I don't want to replace them based on an arbitrary mileage number when runners and surfaces vary so much. Are there useful signs—uneven wear, new aches, flattened foam—that tell you a pair is truly past its best? I'd rather be sensible than either wasteful or stubborn.