The first thing to remember is to always have a good introduction, whether formal or not, you need to introduce the letter with "Dear", or "To Whom It May Concern", or a greeting of sorts for the person that the letter is addressed to. Remember, this is the start of a letter, and doesn't need to be long, just keep it simple and traditional. The second thing to remember in regards to writing in a letter format is to communicate your thoughts clearly. It might take a few drafts to get all the information out and in a concise manner. You have to keep in mind that you want an audience for your writing, and that their time is precious. Do not get too longwinded; so make sure you hit the points you need to hit with concise speech and rhetoric. Start with an introduction paragraph that describes what you're going to talk about in the following writing, just do not make it too vague or too detailed, and find a happy medium. Another good tip is in regards to the body of the letter format. The body should have an informal touch to it, as if you're talking to someone that you know well. If you're giving instructions, asking for information, or anything that is not necessarily too formal, keep it friendly, light and to the point. Remember, you're writing a letter not necessarily a novel.
Only in the event that you already know the recipient should you consider alternatives such as "Kind regards," or "With best wishes," appropriate. If in doubt, stick to "faithfully," or "sincerely,". Also note that in either case, the "Y" of "Yours" is capped, and the "f" or "s" of "faithfully," or "sincerely," respectively is lower_case. Note the comma after both "faithfully," and "sincerely," too. The positioning of the closing phrase differ depending on the format: FBF: Align left, MBF: Align right.