i think the main point is this letter should not exceed a page or more than three paragraphs. yes, you are in essence selling yourself/sucking up but it should not be written in that tone. you should convey what you are doing now,planning to do and how you are a fit for their school. personally i would review their mission statement right before writing how you're fit. be concise, nobody wants to sift through flowery language and bs, what this letter should do is say to the adcom, hey lets pull this guy/girls file and take another look.
there are a few other threads , you can try searching LOI/letter of interest, being aware that LOI can also mean letter of intent.