First off, I've never heard of anyone improving their score from a 25 to a 35. Most schools seem to look at the most recent score, some look at the highest, and fewer still will combine the highest score in each section to produce a new composite score. Search for previous threads to find a breakdown of how schools treat multiple scores, as it's been discussed to death on this forum (e.g., search google for "multiple scores site:studentdoctor.net").
The best advice is to make sure you do well the first time. I actually had to take the test twice, but it was because my previous scores had expired. I don't think it hurt me at all, but it may have if my first score wasn't solid. If your first score is significant below 30, I would suggest a) finding out what happened, and b) doing something else for a little while to make the rest of your application stellar. Though people certainly get accepted with scores below 30, it isn't the norm. At the higher end schools, the median scores of accepted students is approaching 35, with a 10 in each section being the "recommended" lower end.