Long post warning!
1. Read and understand the prompt (if any is given).
2. Formulate a thesis (essentially a knee-jerk reaction to the prompt)
3. Corroborate your thesis with examples, evidence, logic, etc.
4. Conclude the essay, making sure to affirm the value of the prompt

This is the method for composing essays not only for scholarships but for school assignments.

Tips:
A 500-word essay limit should be about 5 paragraphs long.

Find your voice: formal or informal. An essay about a current issue (social, political, etc.) should be formal, that is, NO FIRST PERSON! Use third person in a formal essay. (Note: this post is mainly about formal essays)

Use a word processor. Microsoft Word 2007-2010 is useful for tracking how many words you are using and includes a spellcheck and thesaurus.

Don't use a word if you don't know what it EXACTLY means. Consult a dictionary if not sure.

Formal essays often require citation of sources. Research the latest MLA, APA, and Chicago-style citation methods. Son of Citation is a useful tool for the lazy, but make sure it's accurate!

Research may be required for an essay, especially if it's a topic you're not well-read on. Be careful what sources you use on the internet; the best suffix to look for is .edu. This means it is a collegiate or educational website.

If a sentence is giving you trouble, rewrite or scrap if needed.

If you want to express your opinion without first person, consider using the subject "one." Example: "One might wonder why Noah Webster decided to create a whole new dialect of English for the United States." This expresses a thought or emotion you felt without saying you actually thought or felt it.

Sorry I haven't had any luck yet. I wish I did.

BEING HONEST :)

be passionate of what you're writign about

I'm wondering the same thing:/

I'd say the most you can do is make it very personal. There's no research required when you're writing your own story. Write about something you're passionate about, preferably something that involves helping people. If you do that, you can get into schools like Princeton; who really weigh in your personal essay, and your GPA of course :)

Don't give up. :)

Say something that only you can say. Yes, everyone who applies for a scholarship needs the money. But what do you have to offer that's different, that's unique?

Everyone has a story. Just present yours in a way that is easy to understand and worth funding.

Kind of like college essay writing, I think strong emotion or even humor is a real winner with essay judges. You want to make your essay attractive, so eloquence and conciseness are very important.

When I was writing college essays I was told that the one thing you DO NOT want to do is be negative. If you've had some hardship that you want to share in your essay, that is fine as long as there is a silver lining that tells the essay readers that you've overcome your hardship or that you are working on it and that you are looking forward to the future and want to be able to make something of yourself... (if only you had the money to do so). Basically, always end on a high note.