When you know the recipient’s name, sign it “Sincerely yours” or just “Sincerely”, or in the US “Yours truly” is acceptable. Don’t try anything more cutesy or friendly unless a) you know the recipient quite well and are certain you can be informal without seeming disrespectful, or b) you have a “trademark” letter ending that’s part of your public persona (and the talent and popularity to pull it off). Signature and printed name: Signing your name is a minimal assurance that you stand behind what was written.
Letter Text: Business letter text should be clean and readable. Avoid writing your letter in one large block of text. Break your text into several concise paragraphs. These paragraphs should be aligned to the left; this allows for easier reading. When you have completed your letter, ask someone else to read it for you. Have them glance briefly at the letter. Is there too much text on the page? Is it easy to see the distinct paragraphs?