Thank you guys. That's very helpful!

"Sincerely" is *always* safe; "Sincerely yours" may be slightly more formal; "Yours truly" is also always safe. If you are writing to someone with whom you are acquainted, but you still wish to be a little formal, "Yours very truly", "With best wishes", or "With best regards" would all be totally acceptable. In emails, I will usually use simply "Best wishes" or "Best regards". English speakers of different ages and from different places may have different opinions, but I typically would not sign a letter with just "Yours" -- it strikes me as slightly rude, as though I couldn't be bothered to take the time to type "Yours truly" or "Yours sincerely". I only write "Yours" when I am deliberately trying to be impolite (for example, to a business colleague who does not like me and whom I do not like) but not rude enough to cause serious offense.

Sorry, posted in comments by accident: Well, I assume you are writing a formal letter, correct? Say, to a business, or an important individual? I mean, you would not necessarily use this ending when writing to a friend. If you know the individual's name, you can end with sincerely, so: Dear John/Dear Mr Smith/Dear Miss Smith/Dear Mrs Smith/Dear Susan, etc. are all letters that can end with 'yours sincerely' and then your name two spaces below. If, however, you do not know the name of the person to whom you are writing, so you start 'Dear Sir/Madam, or 'To whom it may concern,' you would end with 'yours faithfully' and then your name.

Well, I assume you are writing a formal letter, correct? Say, to a business, or an important individual? I mean, you would not necessarily use this ending when writing to a friend. If you know the individual's name, you can end with sincerely, so: Dear John/Dear Mr Smith/Dear Miss Smith/Dear Mrs Smith/Dear Susan, etc. are all letters that can end with 'yours sincerely' and then your name two spaces below. If, however, you do not know the name of the person to whom you are writing, so you start 'Dear Sir/Madam, or 'To whom it may concern,' you would end with 'yours faithfully' and then your name.