1. Decide basically on day 1 of MS-1 that you want to do Plastics, then....
Rock Step I (at least 250)
Do research and get at least 1 first author publication in PRS or Annals by the time applications are due
Cure cancer
Be a concert pianist, triathlete, published science fiction author, or something similar to demonstrate you are "multi-talented"
2. If, like the vast majority of us, you missed Step 1, then you need to get some research done as early as possible (get plugged in with your plastics residents, faculty, etc and see if you can piggyback on one of their projects - your draw is that you are motivated and have more free time than they do, so you can do the background research for a case report for example), honor as many MS-3 rotations as you can (obviously including surgery), and set up home and away rotations in plastics. You need some letters from people in the field that say hey, this is a good guy. Plastics is a relatively small field and those letters go a long way.
3. If, like a not small portion of us, you have decided you want to do Plastics a little late for any of this to apply, you're gonna have to take the longer route. Residencies in ENT, surgery, or even OMFS can serve as entries into plastics . Yes, this means you will need a few more years of training, but you will also get arguably a more solid foundation and the opportunity to be double boarded.
It's a great field. People will line up to talk you out of doing it. Don't listen.