I would agree with almost everyone here and say wait a while. Start brainstorming if you feel like it, but don't actually sit down to write it because if another idea comes along you want to be completely open to it... I wrote the first draft of my personal statement about 2 months before my AMCAS went in, and I thought it was golden... So when those who read my ps thought I needed to re-work it, it was REALLY REALLY hard to let go of my original idea. And honestly, my finalized idea really only hit me after a conversation with my mom literally a week before I submitted. So I would suggest spending the summer getting some good clinical and/or research experiences, and maybe writing down ideas for your ps, and then when you have a bunch, bounce them off some people. That way, you don't spend all the energy writing something that might not be as great as you thought, as in my case. And one final word of warning, you will get sick of the thing. I wrote somewhere around 25 drafts of mine, but number 25 said so much more about me than did the first. So good luck!!!