Many designers look at their work and think, “It’s still missing something.” But maybe it’s actually that something needs to be missing. Your design is done when there is nothing left to delete, nothing left to take away. You should start with addition (put everything in your design that you think you need), but to get to the end you must think in subtraction.
It’s what I like to call “Design Math” (oxymoron, right?), add then subtract, and you’ll get a perfect answer to your design problem. A great design is one that you can look at and realize that not one more thing could be taken away to make it any better. That’s when you know you can stop working.