Never use “really” or “very”.
You sound too enthusiastic.

Example:
It was really great to meet you.
vs.
It was great to meet you.

Never use “think”. Use “believe” instead.
If you don’t believe it, don’t write it.

Example:
I think it’s a good idea.
vs.
I believe it’s a good idea.

Don’t apologize (unless you actually owe them an apology).
If you’re so sorry, why are you doing it?

Example:
Sorry to bother you, but I have a quick question.
vs.
I have a quick question.