I've mostly heard people on this forum saying that you don't need to do anything for waitlist emails other than let them know you're still interested in the program, but I did do something a little different last year when I was waitlisted for UVA (albeit, I was still eventually rejected so take my advice with a grain of salt). Per my professor's advice, I used the waitlisted email as an opportunity to share with the DGS what I was able to accomplish in the time since turning in my application. I wouldn't necessarily do that for little things, but during the couple month period in which my application was being considered I had been nominated for a prestigious award at my university. If you've gotten a big award or have been accepted for a publication and it's not included in your application, I would definitely take the waitlist email as an opportunity to share that achievement. Worst case, it does nothing. Best case, it could move you a couple of spots up the waitlist!