Expressions for make a good writing

-Beging with:

Hello/ Hi (informal)          My dear/ Dear...  (formal)

-Opening sentence:

I'm writing to tell you about...     
Thanks for your last e-mail.
Sorry I haven't written for a long time.
It was great to hear from you.
How are you?
How are things?

-Advicing:

You should
The best thing to do is...

-Closing sentences:

Hope to hear from you soon    (Informal)
I look forward to hear from you without delay    (Formal)

-Closing with:

Formal
Yours faithfully,
Your sincerelly, 

Informal
Best wishes for you,
By for now,
All, the best,