I find that "Thanks" or "Many thanks" is useful as a polite but fairly informal sign-off. I use it in emails and sometimes on post-it notes.
The next stage up in formality is "Best wishes" (or sometimes "Best regards" or "Kind regards"). These are fine for polite, relatively formal emails.
"Yours sincerely" (which you use when the addressee is addressed with their name e.g. Dear Mr Smith) and "Yours faithfully" (used where address the person without a name e.g. Dear Sir) are both too formal, even for emails. We only use them in formal letters.