OP, be careful to whom you send your PS. Look around the forums for the of the guys that had been around the SDN for a while and got accepted into dental schools because some of the "volunteers" could just steel your PS.
As people already said, take it to your University writing centre, some of your profs, maybe your english TA (my TA proofread a couple of science lab reports for me).
Also giving it to people that know you (i.e volunteer coordinator, prof, dentist, etc) might help a lot as they might give opinions on you that you never thought of before.
BTW: Do NOT send it too me, I just finished my 2nd year and haven't even done my DAT yet.