No one is a perfect communicator. Even the expert get a lot wrong a lot of the time, because communication is a complex, dynamic skill, deployed in fast-moving settings, with infinite variables, and requiring response or parry to incoming contributions fired at point-blank range.
Nonetheless, there are some fairly well-established dos and don'ts I find it helpful to work towards and away from, respectively:
What does help with communication:
Say what you mean
Mean what you say
But don’t say it mean
Remain cordial
Keep the voice low and relaxed
Speak slowly to lower the temperature
Think very carefully before each thing you say
Be certain it is absolutely true
Be certain is it quite relevant
Be certain it is purposeful and timely
Be certain you are the one to say it
Make the contribution as detailed as necessary
Make the contribution no more detailed than necessary
Pause as necessary
Signal the purpose of the pause: to think before responding
Keep strictly to the topic at hand until the topic is exhausted
Hear the other person out
Interrupt only to halt broadcasting or remedy diversion
What does not help two-way communication:
Ambiguity
Attack
Changing the subject
Circling back
Concealment
Defence
Deflection
Eliciting pity
Evasion
Excessive detail
Excuses
Explanation not explicitly sought
Expressing past or future intentions
Fawning / flattery
Flooding
Grudging concession
Half-truths
Hyperbole
Incoherence
Inconsistency
Insincere engagement
Irrelevance
Kowtowing
Lying
Non sequitur
Non-engagement
Obsequiousness
Perfunctory engagement
Punishment
Responding to presumed subtext
Ridicule
Sighing
Silence other than to pause
Surly / sneering deference
Tangential answers
Terseness
Underplaying
Unfair caricature
Universalised concession (‘Doesn’t everyone …?’)
Unwarranted interruption
Verbosity