Assertiveness is about stating what you need while considering the wants and needs of others. 

Assertiveness also means saying "no" when you need to.

It can be tempting to stereotype new colleagues or clients, or to make assumptions about them based on just a few pieces of information.

Set time aside to listen when you meet someone new. Give them space to talk about their viewpoints and take time to absorb these.

Strong emotional reactions can damage your reputation. 

Instead, try and respond calmly.

Poorly-prepared presentations, reports, or emails frustrate your audience and can, over time, damage your reputation.

• Set aside time to plan your communication thoroughly.
• Create a credible, intelligent, and compelling message that emotionally and intellectually appeals ...

Spelling, tone and grammatical mistakes can make you look careless.

• Don't rely on spell-checkers.
• Proofread your work.
• Use a dictionary to look up any words that you're unsure about. 
• Reading your work aloud makes it easier to catch typos and tone errors....

Always take time to check that people have understood your message.

To check that you've been understood correctly, use open questions that start with "how," "why" or "what." These encourage reflection and will help your audience to explain what they have taken from your commu...

Forwarding sensitive email to the wrong person, or sending an incorrect attachment can cause serious problems.

It might be helpful to draft emails in a word processing document or blank email, and then to paste the text into a new message. Only then select the recipient.

It's tempting to try to avoid difficult conversations, but this can cause further problems.

• Preparation is key to handling difficult conversations.
• Use tools such as the Situation – Behavior – Impact technique to encourage your people to reflect on their behavior.