You always start with addition (put everything in your design that you think you need), but to get to the end you must think in subtraction.
Many designers look at their work and think, “It’s still missing something.” But maybe it’s actually that something needs to be missing. Just before you decide that you’re finished, look at your design and ask what else you could do without. Your design is done when there is nothing left to delete, nothing left to take away.
It’s like 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.