What informal and formal letter/e-mail closings are used the most? [duplicate]

I often struggle with doubts about the correctness of the closings which I use. I'm not a native speaker and I'm worried that I'll make a mistake in the last part of the letter/e-mail.

Some examples:

Formal:

I look forward to hearing from you soon.
If you need any more information, please let me know

Informal:

I hope to hear from you soon.
If you need any more info, just drop me a line.

Could you let me know which closings are used the most?

These are fine, you are correct about the relative formality of the closings.

However, to preserve the formality in the first I would change it to "I look forward to...".