Closing for a letter when I don't want to say "Sincerely" or "Regards"

[Black]

Hasta la Bye Bye,

[Wailer]

Haha. Some really witty suggestions here.

I'd just leave it at 'Sincerely' or 'very truly yours' which both sound very friendly, but actually mean "I'm very serious about what I am saying" as a valediction. My lawyer uses 'very truly yours' when he's very serious but I think it's condescending. 'Sincerely' is probably most appropriate for formal correspondence.

And you should always use a valediction when formally corresponding.

[Yoyodyne ArtWorks]

I would go with "Awaiting your reply" so that they know you expect action. As Wailer points out, "Sincerely" is the best for formal correspondence in general, but in this case you want to leave them with the thought that you expect to hear back from them.