waitlisted twice

Don't give up all hope just yet simply because you're on the WL. If I were you I wouldn't start thinking about next year's applications until much much later. I'm all for being prepared but there's still a lot you can do this year!

You should start by really reviewing all your applications; try to understand why you were dinged (were there any gaps, did you do a good enough job demonstrating your 'fit' with the school) and then see if you've made similar mistakes in your applications that got you on to the WL. Seeing as you made it to the WL, you probably haven't raised any red flags (e.g. GMAT, GPA etc.) and I'm guessing you've most likely been interviewed. That means it most likely about how unique you seem or perhaps your fit with the school. Make sure you really understand the qualities the school is looking for in its students and see if those qualities came out in your essays. If not, think about how you could get another recommendation or other evidence that shows you have those traits that were missing. Uniqueness is probably a bit harder to address; you should just make sure that your application was complete in terms of both what you've done and what you learnt from those experiences.

If you really believe your applications are perfect, and they contain everything that the adcom's want to know about you, then I would spend a little time writing a letter explaining why that school is the right one for you (and why you are right for that school). If you've had the opportunity to visit the schools then it might be a good idea to talk about the things that you saw or experienced and how that made you certain is was the school for you. Otherwise, maybe try to get in touch with some current students and find out a bit more about life at the school before writing the letter.

At the end of the day, if do a good job with whatever you send in it's not going to hurt your application, and a bit of extra work now might save you the hassle of applying again next year.