It's also a good idea to choose a thematic element that that demonstrates a key feature of YOU as an applicant and potential physician. Teachability, diligence, steadfast commitment in the face of adversity-all of these are great.
We recommend Dr. Atul Gawande's "Better" as a good example of writing on Diligence, for example, as a key feature of successful skill building in procedural specialties.
Pretty much what Gonnif said. But I'd also add sincerity. There are a million different reasons to go into medicine and the "I want to help people" tagline gets overused unless you can show that it is your motivator by listing examples and activities.