Why you should dedicate the first paragraph to the receiver and the second paragraph to letting the reader know what you want.
A nice article on SEOmoz covers effective email writing – unlike most such articles, it's not specific to, say, business sales emails, but outlines some general principles which you can include in any type of email. How to Write Email to Get a Better Response Rate offers some decent tips which might apply as much to requesting something from a colleague. It sums up the need to use catchy subject titles and personal names, as well as dedicating the first paragraph to the receiver, the second paragraph to letting the reader know what you want and the need to sum up the email nicely in a courteous way. I've just made a note of this on my jotter and will be trying to stick to it where possible.