I used to keep them all in files. Now I just check off a list when I get a response. That said, when I sold my book, I threw a party and decorated with of all the rejections I had at that point (about 200, for about 6 different novels, some long since abandoned, others rewritten into other stories.). I papered the walls with them. Let me tell you, it was quite a conversation starter. Some people, reading through, actually got angry! One friend called it the "wailing wall." A few friends were over before the party, and we put them up by taping them together in rows of 10, and then hanging the row from the ceiling. After a living room wall was completely covered, we stood back and sort of stared. I broke the silence by saying, "I really wanted to publish a book." the friend said, "We can see that." It was a great moment. A sense of humor helps.