Looking businesslike is the first step to acting businesslike, and people will take you more seriously if you are well-groomed. This also applies to body language, so always make eye contact and keep good posture, or you may appear to lack confidence.
Make it personal
Remember people's names and they will remember you. Do everything you can to avoid addressing a message "Dear Sir/Madam". Remember that business is about building relationships.
Be formal but not stiff
If you are too formal, people won't be at ease around you. Act too casual and people may not take you seriously. Aim for the middle ground!
Be clear and concise
Get to the heart of the matter quickly and don't waste words. On the other hand, don't assume that the person you're talking to knows who you are and why you are contacting them - they may need a reminder and some background.
Reply to correspondence quickly
Acknowledge receipt of important emails and phone calls, and if you'll be away for longer than a day, let people know you'll be out of the office.
Keep your cool
In any job, difficult situations will pop up. Losing your temper won't help and will just complicate things. Show that you can handle stress, and people will take notice and give you more responsibility in the future.
Communication is a two way street. Listen to the opinions of others, don't interrupt people when they are speaking, and don't ignore criticism. We all make mistakes and our colleagues often have good suggestions. Also, people will be more willing to listen to you if they see that you're listening to them!
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