Basically, what Nick is saying. Blu- Er, embellish as much as possible. Make yourself sound impressive, but also make yourself sound like you need the scholarship.
After being rejected for automatic scholarships due to .02 points, I emailed the university employees and head of my undergraduate program several times asking for their help. The guy promised to look into it, and here I am today.
Don't just contact the university, or the office that gets hundreds of similar letters every year. Find someone on the inside, and light a fire under their ass. People that only deal with money will be stingier.
EDIT: Oh, and be somewhat concise. Pick a sentence that hits the person in the face with your greatness, and keep punching. Nobody wants to slog through a long letter that takes forever to get to the point.
Honestly, writing like this is a value life skill. College will do everything it can to reach it to you, but being ahead of the curve is useful.