I'm assuming you mean you ARE on the path of starting? Haha
I think one of the biggest things is to show not tell. Do not just list achievements or things that have happened in your life. Pick a few experiences that have really shaped who you are and how they led you to medicine and then show how they taught you what they did. Again, to reemphasize, show don't tell. Some of the best advice I got was "you have to get so passionate and emotional in your writing that it almost makes you sick, then you know you're there." When other people read your PS, they don't see it as overly mushy and obnoxious, they see that you're passionate. It'll feel almost over the top to you but it doesn't come across that way. Once you have a few experiences that you want to write about, just start writing. Seriously, just get something down on paper even if it's just word vomit trying to get ideas out there. Then you can edit it and adjust things. Make sure to get lots of eyes on it!! Have your adviser, your mom, your other pre-health friends, your neighbor, read it and give you feed back. Everyone has a different perspective and it's nice to get lots of input on it. It's going to get torn apart and rebuilt over and over so just be ready for that. Sometimes it hurts to hear what others say because you worked hard on it but don't take it personal. With that, make sure to keep it your own and don't let someone else write it but take their advice seriously and be open to criticism. I tried to have an underlying theme to mine but I'm not sure if that's something everyone does.
Hope that helps and makes sense. I kind of just jotted down thoughts as they came. Good luck!